--- a/layout/tools/reftest/moz.build
+++ b/layout/tools/reftest/moz.build
@@ -15,17 +15,16 @@ FINAL_TARGET_PP_FILES += ['install.rdf']
 FINAL_TARGET_FILES += ['bootstrap.js']
 
 TEST_HARNESS_FILES.reftest += [
     '/build/mobile/remoteautomation.py',
     '/build/pgo/server-locations.txt',
     '/testing/mochitest/server.js',
     'mach_test_package_commands.py',
     'output.py',
-    'reftest-preferences.js',
     'reftestcommandline.py',
     'remotereftest.py',
     'runreftest.py',
 ]
 
 TEST_HARNESS_FILES.reftest.chrome += [
     'chrome/binding.xml',
     'chrome/userContent.css',

--- a/layout/tools/reftest/runreftest.py
+++ b/layout/tools/reftest/runreftest.py
@@ -277,19 +277,71 @@ class RefTest(object):
         :param profile_to_clone: Path to a profile to use as the basis for the
                                  test profile
         :param prefs: Extra preferences to set in the profile
         """
         locations = mozprofile.permissions.ServerLocations()
         locations.add_host(server, scheme='http', port=port)
         locations.add_host(server, scheme='https', port=port)
 
-        # Set preferences for communication between our command line arguments
-        # and the reftest harness.  Preferences that are required for reftest
-        # to work should instead be set in reftest-preferences.js .
+        sandbox_whitelist_paths = options.sandboxReadWhitelist
+        if (platform.system() == "Linux" or
+            platform.system() in ("Windows", "Microsoft")):
+            # Trailing slashes are needed to indicate directories on Linux and Windows
+            sandbox_whitelist_paths = map(lambda p: os.path.join(p, ""),
+                                          sandbox_whitelist_paths)
+
+        addons = []
+        if not self.use_marionette:
+            addons.append(options.reftestExtensionPath)
+
+        if options.specialPowersExtensionPath is not None:
+            if not self.use_marionette:
+                addons.append(options.specialPowersExtensionPath)
+
+        # Install distributed extensions, if application has any.
+        distExtDir = os.path.join(options.app[:options.app.rfind(os.sep)],
+                                  "distribution", "extensions")
+        if os.path.isdir(distExtDir):
+            for f in os.listdir(distExtDir):
+                addons.append(os.path.join(distExtDir, f))
+
+        # Install custom extensions.
+        for f in options.extensionsToInstall:
+            addons.append(self.getFullPath(f))
+
+        kwargs = {'addons': addons,
+                  'locations': locations,
+                  'whitelistpaths': sandbox_whitelist_paths}
+        if profile_to_clone:
+            profile = mozprofile.Profile.clone(profile_to_clone, **kwargs)
+        else:
+            profile = mozprofile.Profile(**kwargs)
+
+        # First set prefs from the base profiles under testing/profiles.
+        profile_data_dir = os.path.join(SCRIPT_DIRECTORY, 'profile_data')
+
+        # If possible, read profile data from topsrcdir. This prevents us from
+        # requiring a re-build to pick up newly added extensions in the
+        # <profile>/extensions directory.
+        if build_obj:
+            path = os.path.join(build_obj.topsrcdir, 'testing', 'profiles')
+            if os.path.isdir(path):
+                profile_data_dir = path
+
+        with open(os.path.join(profile_data_dir, 'profiles.json'), 'r') as fh:
+            base_profiles = json.load(fh)['reftest']
+
+        for name in base_profiles:
+            path = os.path.join(profile_data_dir, name)
+            profile.merge(path)
+
+        # Second set preferences for communication between our command line
+        # arguments and the reftest harness. Preferences that are required for
+        # reftest to work should instead be set under srcdir/testing/profiles.
         prefs = prefs or {}
         prefs['reftest.timeout'] = options.timeout * 1000
         if options.logFile:
             prefs['reftest.logFile'] = options.logFile
         if options.ignoreWindowSize:
             prefs['reftest.ignoreWindowSize'] = True
         if options.shuffle:
             prefs['reftest.shuffle'] = True
@@ -301,16 +353,25 @@ class RefTest(object):
             prefs['reftest.verify'] = True
         if options.cleanupCrashes:
             prefs['reftest.cleanupPendingCrashes'] = True
         prefs['reftest.focusFilterMode'] = options.focusFilterMode
         prefs['reftest.logLevel'] = options.log_tbpl_level or 'info'
         prefs['reftest.suite'] = options.suite
         prefs['reftest.runSlower'] = options.runSlower
 
+        # Set tests to run or manifests to parse.
+        if tests:
+            testlist = os.path.join(profile.profile, 'reftests.json')
+            with open(testlist, 'w') as fh:
+                json.dump(tests, fh)
+            prefs['reftest.tests'] = testlist
+        elif manifests:
+            prefs['reftest.manifests'] = json.dumps(manifests)
+
         # Unconditionally update the e10s pref.
         if options.e10s:
             prefs['browser.tabs.remote.autostart'] = True
         else:
             prefs['browser.tabs.remote.autostart'] = False
 
         if not self.run_by_manifest:
             if options.totalChunks:
@@ -318,86 +379,42 @@ class RefTest(object):
             if options.thisChunk:
                 prefs['reftest.thisChunk'] = options.thisChunk
 
         # Bug 1262954: For winXP + e10s disable acceleration
         if platform.system() in ("Windows", "Microsoft") and \
            '5.1' in platform.version() and options.e10s:
             prefs['layers.acceleration.disabled'] = True
 
-        sandbox_whitelist_paths = options.sandboxReadWhitelist
-        if (platform.system() == "Linux" or
-            platform.system() in ("Windows", "Microsoft")):
-            # Trailing slashes are needed to indicate directories on Linux and Windows
-            sandbox_whitelist_paths = map(lambda p: os.path.join(p, ""),
-                                          sandbox_whitelist_paths)
-
         # Bug 1300355: Disable canvas cache for win7 as it uses
         # too much memory and causes OOMs.
         if platform.system() in ("Windows", "Microsoft") and \
            '6.1' in platform.version():
             prefs['reftest.nocache'] = True
 
         if options.marionette:
             # options.marionette can specify host:port
             port = options.marionette.split(":")[1]
             prefs["marionette.port"] = int(port)
 
         # Enable tracing output for detailed failures in case of
         # failing connection attempts, and hangs (bug 1397201)
         prefs["marionette.log.level"] = "TRACE"
 
-        preference_file = os.path.join(here, 'reftest-preferences.js')
-        prefs.update(mozprofile.Preferences.read_prefs(preference_file))
-
+        # Third, set preferences passed in via the command line.
         for v in options.extraPrefs:
             thispref = v.split('=')
             if len(thispref) < 2:
                 print "Error: syntax error in --setpref=" + v
                 sys.exit(1)
             prefs[thispref[0]] = thispref[1].strip()
 
-        addons = []
-        if not self.use_marionette:
-            addons.append(options.reftestExtensionPath)
-
-        if options.specialPowersExtensionPath is not None:
-            if not self.use_marionette:
-                addons.append(options.specialPowersExtensionPath)
-
         for pref in prefs:
             prefs[pref] = mozprofile.Preferences.cast(prefs[pref])
-
-        # Install distributed extensions, if application has any.
-        distExtDir = os.path.join(options.app[:options.app.rfind(os.sep)],
-                                  "distribution", "extensions")
-        if os.path.isdir(distExtDir):
-            for f in os.listdir(distExtDir):
-                addons.append(os.path.join(distExtDir, f))
-
-        # Install custom extensions.
-        for f in options.extensionsToInstall:
-            addons.append(self.getFullPath(f))
-
-        kwargs = {'addons': addons,
-                  'preferences': prefs,
-                  'locations': locations,
-                  'whitelistpaths': sandbox_whitelist_paths}
-        if profile_to_clone:
-            profile = mozprofile.Profile.clone(profile_to_clone, **kwargs)
-        else:
-            profile = mozprofile.Profile(**kwargs)
-
-        if tests:
-            testlist = os.path.join(profile.profile, 'reftests.json')
-            with open(testlist, 'w') as fh:
-                json.dump(tests, fh)
-            profile.set_preferences({'reftest.tests': testlist})
-        elif manifests:
-            profile.set_preferences({'reftest.manifests': json.dumps(manifests)})
+        profile.set_preferences(prefs)
 
         if os.path.join(here, 'chrome') not in options.extraProfileFiles:
             options.extraProfileFiles.append(os.path.join(here, 'chrome'))
 
         self.copyExtraFilesToProfile(options, profile)
         return profile
 
     def environment(self, **kwargs):

--- a/testing/profiles/moz.build
+++ b/testing/profiles/moz.build
@@ -3,16 +3,18 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 profile_files = [
     'common/*',
     'perf/*',
     'profiles.json',
+    'reftest/*',
     'unittest/*',
 ]
 
+TEST_HARNESS_FILES.reftest.profile_data += profile_files
 TEST_HARNESS_FILES.testing.mochitest.profile_data += profile_files
 TEST_HARNESS_FILES['web-platform'].prefs += profile_files
 
 with Files("**"):
     BUG_COMPONENT = ("Testing", "General")

--- a/testing/profiles/profiles.json
+++ b/testing/profiles/profiles.json
@@ -1,8 +1,9 @@
 {
     "mochitest": ["common", "unittest"],
     "profileserver": ["common", "unittest"],
     "raptor": ["common", "perf", "raptor"],
+    "reftest": ["reftest"],
     "talos": ["common", "perf"],
     "valgrind": ["common", "unittest"],
     "web-platform-tests": ["common", "unittest"]
 }

new file mode 100644
--- /dev/null
+++ b/testing/profiles/reftest/extensions/README.txt
@@ -0,0 +1,2 @@
+Dropping extensions here will get them installed in all test harnesses
+that make use of this profile.

rename from layout/tools/reftest/reftest-preferences.js
rename to testing/profiles/reftest/user.js