--- a/build/moz.configure/init.configure
+++ b/build/moz.configure/init.configure
@@ -83,17 +83,16 @@ add_old_configure_assignment(
     'DIST', check_build_environment.dist)
 
 option(env='MOZ_AUTOMATION', help='Enable options for automated builds')
 set_config('MOZ_AUTOMATION', depends_if('MOZ_AUTOMATION')(lambda x: True))
 
 
 option(env='OLD_CONFIGURE', nargs=1, help='Path to the old configure script')
 
-option(env='MOZ_CURRENT_PROJECT', nargs=1, help='Current build project')
 option(env='MOZCONFIG', nargs=1, help='Mozconfig location')
 
 option('--with-external-source-dir', env='EXTERNAL_SOURCE_DIR', nargs=1,
        help='External directory containing additional build files')
 
 
 @depends('--with-external-source-dir')
 def external_source_dir(value):
@@ -106,21 +105,21 @@ add_old_configure_assignment('EXTERNAL_S
 
 # Read user mozconfig
 # ==============================================================
 # Note: the dependency on --help is only there to always read the mozconfig,
 # even when --help is passed. Without this dependency, the function wouldn't
 # be called when --help is passed, and the mozconfig wouldn't be read.
 
 
-@depends('MOZ_CURRENT_PROJECT', 'MOZCONFIG', 'OLD_CONFIGURE',
+@depends('MOZCONFIG', 'OLD_CONFIGURE',
          check_build_environment, '--with-external-source-dir',
          '--help')
 @imports(_from='mozbuild.mozconfig', _import='MozconfigLoader')
-def mozconfig(current_project, mozconfig, old_configure, build_env,
+def mozconfig(mozconfig, old_configure, build_env,
               external_source_dir, help):
     if not old_configure:
         die('The OLD_CONFIGURE environment variable must be set')
 
     # Don't read the mozconfig for the js configure (yay backwards
     # compatibility)
     # While the long term goal is that js and top-level use the same configure
     # and the same overall setup, including the possibility to use mozconfigs,
@@ -138,20 +137,19 @@ def mozconfig(current_project, mozconfig
     # old-configure dies.
     if os.path.dirname(os.path.abspath(old_configure[0])).endswith('/js/src'):
         return {'path': None}
 
     topsrcdir = build_env.topsrcdir
     if external_source_dir:
         topsrcdir = external_source_dir[0]
     loader = MozconfigLoader(topsrcdir)
-    current_project = current_project[0] if current_project else None
     mozconfig = mozconfig[0] if mozconfig else None
     mozconfig = loader.find_mozconfig(env={'MOZCONFIG': mozconfig})
-    mozconfig = loader.read_mozconfig(mozconfig, moz_build_app=current_project)
+    mozconfig = loader.read_mozconfig(mozconfig)
 
     return mozconfig
 
 
 set_config('MOZCONFIG', depends(mozconfig)(lambda m: m['path']))
 
 
 option(env='PYTHON', nargs=1, help='Python interpreter')

--- a/client.mk
+++ b/client.mk
@@ -17,18 +17,16 @@
 #    build
 #    clean (realclean is now the same as clean)
 #    distclean
 #
 # See http://developer.mozilla.org/en/docs/Build_Documentation for 
 # more information.
 #
 # Options:
-#   MOZ_BUILD_PROJECTS   - Build multiple projects in subdirectories
-#                          of MOZ_OBJDIR
 #   MOZ_OBJDIR           - Destination object directory
 #   MOZ_MAKE_FLAGS       - Flags to pass to $(MAKE)
 #   MOZ_PREFLIGHT_ALL    - Makefiles to run before building.
 #   MOZ_POSTFLIGHT_ALL   - Makefiles to run after building.
 #
 #######################################################################
 # Defines
 
@@ -100,18 +98,17 @@ define CR
 endef
 
 # As $(shell) doesn't preserve newlines, use sed to replace them with an
 # unlikely sequence (||), which is then replaced back to newlines by make
 # before evaluation. $(shell) replacing newlines with spaces, || is always
 # followed by a space (since sed doesn't remove newlines), except on the
 # last line, so replace both '|| ' and '||'.
 # Also, make MOZ_PGO available to mozconfig when passed on make command line.
-# Likewise for MOZ_CURRENT_PROJECT.
-MOZCONFIG_CONTENT := $(subst ||,$(CR),$(subst || ,$(CR),$(shell $(addprefix MOZ_CURRENT_PROJECT=,$(MOZ_CURRENT_PROJECT)) MOZ_PGO=$(MOZ_PGO) $(TOPSRCDIR)/mach environment --format=client.mk | sed 's/$$/||/')))
+MOZCONFIG_CONTENT := $(subst ||,$(CR),$(subst || ,$(CR),$(shell MOZ_PGO=$(MOZ_PGO) $(TOPSRCDIR)/mach environment --format=client.mk | sed 's/$$/||/')))
 $(eval $(MOZCONFIG_CONTENT))
 
 export FOUND_MOZCONFIG
 
 # As '||' was used as a newline separator, it means it's not occurring in
 # lines themselves. It can thus safely be used to replaces normal spaces,
 # to then replace newlines with normal spaces. This allows to get a list
 # of mozconfig output lines.
@@ -139,26 +136,16 @@ ifeq (,$(findstring -j,$(MOZ_MAKE_FLAGS)
 endif
 
 ifdef MOZ_AUTOMATION
 ifeq (4.0,$(firstword $(sort 4.0 $(MAKE_VERSION))))
 MOZ_MAKE_FLAGS += --output-sync=line
 endif
 endif
 
-ifdef MOZ_BUILD_PROJECTS
-
-ifdef MOZ_CURRENT_PROJECT
-  BUILD_PROJECT_ARG = MOZ_BUILD_APP=$(MOZ_CURRENT_PROJECT)
-  export MOZ_CURRENT_PROJECT
-else
-  MOZ_MAKE = $(error Cannot build in the OBJDIR when MOZ_CURRENT_PROJECT is not set.)
-endif
-endif # MOZ_BUILD_PROJECTS
-
 MOZ_MAKE = $(MAKE) $(MOZ_MAKE_FLAGS) -C $(OBJDIR)
 
 # 'configure' scripts generated by autoconf.
 CONFIGURES := $(TOPSRCDIR)/configure
 CONFIGURES += $(TOPSRCDIR)/js/src/configure
 
 # Make targets that are going to be passed to the real build system
 OBJDIR_TARGETS = install export libs clean realclean distclean maybe_clobber_profiledbuild upload sdk installer package package-compare stage-package source-package l10n-check automation/build
@@ -172,43 +159,26 @@ build::
 
 # Include baseconfig.mk for its $(MAKE) validation.
 include $(TOPSRCDIR)/config/baseconfig.mk
 
 # Define mkdir
 include $(TOPSRCDIR)/config/makefiles/makeutils.mk
 include $(TOPSRCDIR)/config/makefiles/autotargets.mk
 
-# Create a makefile containing the mk_add_options values from mozconfig,
-# but only do so when OBJDIR is defined (see further above).
-ifdef MOZ_BUILD_PROJECTS
-ifdef MOZ_CURRENT_PROJECT
-WANT_MOZCONFIG_MK = 1
-else
-WANT_MOZCONFIG_MK =
-endif
-else
-WANT_MOZCONFIG_MK = 1
-endif
-
-ifdef WANT_MOZCONFIG_MK
 # For now, only output "export" lines and lines containing UPLOAD_EXTRA_FILES
 # from mach environment --format=client.mk output.
 MOZCONFIG_MK_LINES := $(filter export||% UPLOAD_EXTRA_FILES% %UPLOAD_EXTRA_FILES%,$(MOZCONFIG_OUT_LINES))
 $(OBJDIR)/.mozconfig.mk: $(TOPSRCDIR)/client.mk $(FOUND_MOZCONFIG) $(call mkdir_deps,$(OBJDIR)) $(OBJDIR)/CLOBBER
 	$(if $(MOZCONFIG_MK_LINES),( $(foreach line,$(MOZCONFIG_MK_LINES), echo '$(subst ||, ,$(line))';) )) > $@
-ifdef MOZ_CURRENT_PROJECT
-	echo export MOZ_CURRENT_PROJECT=$(MOZ_CURRENT_PROJECT) >> $@
-endif
 
 # Include that makefile so that it is created. This should not actually change
 # the environment since MOZCONFIG_CONTENT, which MOZCONFIG_OUT_LINES derives
 # from, has already been eval'ed.
 include $(OBJDIR)/.mozconfig.mk
-endif
 
 # Print out any options loaded from mozconfig.
 all realbuild clean distclean export libs install realclean::
 ifneq (,$(strip $(MOZCONFIG_OUT_FILTERED)))
 	$(info Adding client.mk options from $(FOUND_MOZCONFIG):)
 	$(foreach line,$(MOZCONFIG_OUT_FILTERED),$(info $(NULL) $(NULL) $(NULL) $(NULL) $(subst ||, ,$(line))))
 endif
 
@@ -219,19 +189,18 @@ clobber clobber_all: clean
 # helper target for mobile
 build_and_deploy: build package install
 
 # Do everything from scratch
 everything: clean build
 
 ####################################
 # Profile-Guided Optimization
-#  This is up here, outside of the MOZ_CURRENT_PROJECT logic so that this
-#  is usable in multi-pass builds, where you might not have a runnable
-#  application until all the build passes and postflight scripts have run.
+#  This is up here so that this is usable in multi-pass builds, where you
+# might not have a runnable application until all the build passes have run.
 profiledbuild::
 	$(call BUILDSTATUS,TIERS pgo_profile_generate pgo_package pgo_profile pgo_clobber pgo_profile_use)
 	$(call BUILDSTATUS,TIER_START pgo_profile_generate)
 	$(MAKE) -f $(TOPSRCDIR)/client.mk realbuild MOZ_PROFILE_GENERATE=1 MOZ_PGO_INSTRUMENTED=1 CREATE_MOZCONFIG_JSON=
 	$(call BUILDSTATUS,TIER_FINISH pgo_profile_generate)
 	$(call BUILDSTATUS,TIER_START pgo_package)
 	$(MAKE) -C $(OBJDIR) package MOZ_PGO_INSTRUMENTED=1 MOZ_INTERNAL_SIGNING_FORMAT= MOZ_EXTERNAL_SIGNING_FORMAT=
 	rm -f $(OBJDIR)/jarlog/en-US.log
@@ -244,50 +213,23 @@ profiledbuild::
 	$(call BUILDSTATUS,TIER_FINISH pgo_clobber)
 	$(call BUILDSTATUS,TIER_START pgo_profile_use)
 	$(MAKE) -f $(TOPSRCDIR)/client.mk realbuild MOZ_PROFILE_USE=1 CREATE_MOZCONFIG_JSON=
 	$(call BUILDSTATUS,TIER_FINISH pgo_profile_use)
 
 #####################################################
 # Preflight, before building any project
 
+ifdef MOZ_PREFLIGHT_ALL
 realbuild preflight_all::
-ifeq (,$(MOZ_CURRENT_PROJECT)$(if $(MOZ_PREFLIGHT_ALL),,1))
-# Don't run preflight_all for individual projects in multi-project builds
-# (when MOZ_CURRENT_PROJECT is set.)
-ifndef MOZ_BUILD_PROJECTS
-# Building a single project, OBJDIR is usable.
 	set -e; \
 	for mkfile in $(MOZ_PREFLIGHT_ALL); do \
 	  $(MAKE) -f $(TOPSRCDIR)/$$mkfile preflight_all TOPSRCDIR=$(TOPSRCDIR) OBJDIR=$(OBJDIR) MOZ_OBJDIR=$(MOZ_OBJDIR); \
 	done
-else
-# OBJDIR refers to the project-specific OBJDIR, which is not available at
-# this point when building multiple projects.  Only MOZ_OBJDIR is available.
-	set -e; \
-	for mkfile in $(MOZ_PREFLIGHT_ALL); do \
-	  $(MAKE) -f $(TOPSRCDIR)/$$mkfile preflight_all TOPSRCDIR=$(TOPSRCDIR) MOZ_OBJDIR=$(MOZ_OBJDIR) MOZ_BUILD_PROJECTS='$(MOZ_BUILD_PROJECTS)'; \
-	done
 endif
-endif
-
-# If we're building multiple projects, but haven't specified which project,
-# loop through them.
-
-ifeq (,$(MOZ_CURRENT_PROJECT)$(if $(MOZ_BUILD_PROJECTS),,1))
-configure realbuild $(OBJDIR_TARGETS)::
-	set -e; \
-	for app in $(MOZ_BUILD_PROJECTS); do \
-	  $(MAKE) -f $(TOPSRCDIR)/client.mk $@ MOZ_CURRENT_PROJECT=$$app; \
-	done
-
-else
-
-# MOZ_CURRENT_PROJECT: either doing a single-project build, or building an
-# individual project in a multi-project build.
 
 ####################################
 # Configure
 
 MAKEFILE      = $(wildcard $(OBJDIR)/Makefile)
 CONFIG_STATUS = $(wildcard $(OBJDIR)/config.status)
 CONFIG_CACHE  = $(wildcard $(OBJDIR)/config.cache)
 
@@ -340,17 +282,16 @@ endif
 	    $(TOPSRCDIR)/python/mozbuild/mozbuild/controller/clobber.py $(TOPSRCDIR) $(OBJDIR)
 
 configure-files: $(CONFIGURES)
 
 configure-preqs = \
   $(OBJDIR)/CLOBBER \
   configure-files \
   $(call mkdir_deps,$(OBJDIR)) \
-  $(if $(MOZ_BUILD_PROJECTS),$(call mkdir_deps,$(MOZ_OBJDIR))) \
   save-mozconfig \
   $(OBJDIR)/.mozconfig.json \
   $(NULL)
 
 CREATE_MOZCONFIG_JSON = $(shell $(TOPSRCDIR)/mach environment --format=json -o $(OBJDIR)/.mozconfig.json)
 # Force CREATE_MOZCONFIG_JSON above to be resolved, without side effects in
 # case the result is non empty, and allowing an override on the make command
 # line not running the command (using := $(shell) still runs the shell command).
@@ -397,48 +338,30 @@ realbuild::  $(OBJDIR)/Makefile $(OBJDIR
 
 ####################################
 # Other targets
 
 # Pass these target onto the real build system
 $(OBJDIR_TARGETS):: $(OBJDIR)/Makefile $(OBJDIR)/config.status
 	+$(MOZ_MAKE) $@
 
-endif # MOZ_CURRENT_PROJECT
-
 ####################################
 # Postflight, after building all projects
 
 ifdef MOZ_AUTOMATION
-ifndef MOZ_CURRENT_PROJECT
 $(if $(MOZ_PGO),profiledbuild,realbuild)::
-# Only run the automation/build target for the first project.
-# (i.e. first platform of universal builds)
-	$(MAKE) -f $(TOPSRCDIR)/client.mk automation/build $(addprefix MOZ_CURRENT_PROJECT=,$(firstword $(MOZ_BUILD_PROJECTS)))
-endif
+	$(MAKE) -f $(TOPSRCDIR)/client.mk automation/build
 endif
 
+ifdef MOZ_POSTFLIGHT_ALL
 realbuild postflight_all::
-ifeq (,$(MOZ_CURRENT_PROJECT)$(if $(MOZ_POSTFLIGHT_ALL),,1))
-# Don't run postflight_all for individual projects in multi-project builds
-# (when MOZ_CURRENT_PROJECT is set.)
-ifndef MOZ_BUILD_PROJECTS
-# Building a single project, OBJDIR is usable.
 	set -e; \
 	for mkfile in $(MOZ_POSTFLIGHT_ALL); do \
 	  $(MAKE) -f $(TOPSRCDIR)/$$mkfile postflight_all TOPSRCDIR=$(TOPSRCDIR) OBJDIR=$(OBJDIR) MOZ_OBJDIR=$(MOZ_OBJDIR); \
 	done
-else
-# OBJDIR refers to the project-specific OBJDIR, which is not available at
-# this point when building multiple projects.  Only MOZ_OBJDIR is available.
-	set -e; \
-	for mkfile in $(MOZ_POSTFLIGHT_ALL); do \
-	  $(MAKE) -f $(TOPSRCDIR)/$$mkfile postflight_all TOPSRCDIR=$(TOPSRCDIR) MOZ_OBJDIR=$(MOZ_OBJDIR) MOZ_BUILD_PROJECTS='$(MOZ_BUILD_PROJECTS)'; \
-	done
-endif
 endif
 
 echo-variable-%:
 	@echo $($*)
 
 # This makefile doesn't support parallel execution. It does pass
 # MOZ_MAKE_FLAGS to sub-make processes, so they will correctly execute
 # in parallel.

--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -210,18 +210,17 @@ class MozbuildObject(ProcessExecutionMix
     @property
     def mozconfig(self):
         """Returns information about the current mozconfig file.
 
         This a dict as returned by MozconfigLoader.read_mozconfig()
         """
         if not isinstance(self._mozconfig, dict):
             loader = MozconfigLoader(self.topsrcdir)
-            self._mozconfig = loader.read_mozconfig(path=self._mozconfig,
-                moz_build_app=os.environ.get('MOZ_CURRENT_PROJECT'))
+            self._mozconfig = loader.read_mozconfig(path=self._mozconfig)
 
         return self._mozconfig
 
     @property
     def config_environment(self):
         """Returns the ConfigEnvironment for the current build configuration.
 
         This property is only available once configure has executed.

--- a/python/mozbuild/mozbuild/mozconfig.py
+++ b/python/mozbuild/mozbuild/mozconfig.py
@@ -184,17 +184,17 @@ class MozconfigLoader(object):
 
         for path in deprecated_paths:
             if os.path.exists(path):
                 raise MozconfigFindException(
                     MOZCONFIG_LEGACY_PATH % (path, self.topsrcdir))
 
         return None
 
-    def read_mozconfig(self, path=None, moz_build_app=None):
+    def read_mozconfig(self, path=None):
         """Read the contents of a mozconfig into a data structure.
 
         This takes the path to a mozconfig to load. If the given path is
         AUTODETECT, will try to find a mozconfig from the environment using
         find_mozconfig().
 
         mozconfig files are shell scripts. So, we can't just parse them.
         Instead, we run the shell script in a wrapper which allows us to record
@@ -309,20 +309,16 @@ class MozconfigLoader(object):
         filt = lambda x, y: {k: v for k, v in x.items() if k not in y}
         result['vars'] = diff_vars(
             filt(parsed['vars_before'], parsed['env_before']),
             filt(parsed['vars_after'], parsed['env_after'])
         )
 
         result['configure_args'] = [self._expand(o) for o in parsed['ac']]
 
-        if moz_build_app is not None:
-            result['configure_args'].extend(self._expand(o) for o in
-                parsed['ac_app'][moz_build_app])
-
         if 'MOZ_OBJDIR' in parsed['env_before']:
             result['topobjdir'] = parsed['env_before']['MOZ_OBJDIR']
 
         mk = [self._expand(o) for o in parsed['mk']]
 
         for o in mk:
             match = self.RE_MAKE_VARIABLE.match(o)
 
@@ -342,17 +338,16 @@ class MozconfigLoader(object):
 
             result['make_extra'].append(o)
 
         return result
 
     def _parse_loader_output(self, output):
         mk_options = []
         ac_options = []
-        ac_app_options = defaultdict(list)
         before_source = {}
         after_source = {}
         env_before_source = {}
         env_after_source = {}
 
         current = None
         current_type = None
         in_variable = None
@@ -380,19 +375,16 @@ class MozconfigLoader(object):
                 assert not in_variable
                 section = line[len('------END_'):]
                 assert current_type == section
 
                 if current_type == 'AC_OPTION':
                     ac_options.append('\n'.join(current))
                 elif current_type == 'MK_OPTION':
                     mk_options.append('\n'.join(current))
-                elif current_type == 'AC_APP_OPTION':
-                    app = current.pop(0)
-                    ac_app_options[app].append('\n'.join(current))
 
                 current = None
                 current_type = None
                 continue
 
             assert current_type is not None
 
             vars_mapping = {
@@ -469,17 +461,16 @@ class MozconfigLoader(object):
 
                 continue
 
             current.append(line)
 
         return {
             'mk': mk_options,
             'ac': ac_options,
-            'ac_app': ac_app_options,
             'vars_before': before_source,
             'vars_after': after_source,
             'env_before': env_before_source,
             'env_after': env_after_source,
         }
 
     def _expand(self, s):
         return s.replace('@TOPSRCDIR@', self.topsrcdir)

--- a/python/mozbuild/mozbuild/mozconfig_loader
+++ b/python/mozbuild/mozbuild/mozconfig_loader
@@ -19,25 +19,16 @@ ac_add_options() {
       ;;
     esac
     echo "------BEGIN_AC_OPTION"
     echo $_mozconfig_opt
     echo "------END_AC_OPTION"
   done
 }
 
-ac_add_app_options() {
-  _mozconfig_app=$1
-  shift
-  echo "------BEGIN_AC_APP_OPTION"
-  echo $_mozconfig_app
-  echo "$*"
-  echo "------END_AC_APP_OPTION"
-}
-
 mk_add_options() {
   for _mozconfig_opt; do
     echo "------BEGIN_MK_OPTION"
     echo $_mozconfig_opt
     echo "------END_MK_OPTION"
   done
 }
 

--- a/python/mozbuild/mozbuild/test/test_mozconfig.py
+++ b/python/mozbuild/mozbuild/test/test_mozconfig.py
@@ -282,34 +282,16 @@ class TestMozconfigLoader(unittest.TestC
             mozconfig.write('ac_add_options --foo=@TOPSRCDIR@\n')
             mozconfig.flush()
 
             loader = self.get_loader()
             result = loader.read_mozconfig(mozconfig.name)
             self.assertEqual(result['configure_args'], [
                 '--foo=%s' % loader.topsrcdir])
 
-    def test_read_ac_app_options(self):
-        with NamedTemporaryFile(mode='w') as mozconfig:
-            mozconfig.write('ac_add_options --foo=@TOPSRCDIR@\n')
-            mozconfig.write('ac_add_app_options app1 --bar=@TOPSRCDIR@\n')
-            mozconfig.write('ac_add_app_options app2 --bar=x\n')
-            mozconfig.flush()
-
-            loader = self.get_loader()
-            result = loader.read_mozconfig(mozconfig.name, moz_build_app='app1')
-            self.assertEqual(result['configure_args'], [
-                '--foo=%s' % loader.topsrcdir,
-                '--bar=%s' % loader.topsrcdir])
-
-            result = loader.read_mozconfig(mozconfig.name, moz_build_app='app2')
-            self.assertEqual(result['configure_args'], [
-                '--foo=%s' % loader.topsrcdir,
-                '--bar=x'])
-
     def test_read_capture_mk_options(self):
         """Ensures mk_add_options calls are captured."""
         with NamedTemporaryFile(mode='w') as mozconfig:
             mozconfig.write('mk_add_options MOZ_OBJDIR=/foo/bar\n')
             mozconfig.write('mk_add_options MOZ_MAKE_FLAGS="-j8 -s"\n')
             mozconfig.write('mk_add_options FOO="BAR BAZ"\n')
             mozconfig.write('mk_add_options BIZ=1\n')
             mozconfig.flush()