--- a/mail/app.mozbuild
+++ b/mail/app.mozbuild
@@ -1,31 +1,31 @@
 # vim: set filetype=python:
 # 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/.
 
 # Note that paths in this file are relative to the top directory, which may be
 # m-c or c-c.
 
+include('/toolkit/toolkit.mozbuild')
 include('../mailnews/mailnews.mozbuild')
 
-DIRS += ['/%s/mail/components' % CONFIG['commreltopsrcdir']]
-
-include('/%s/toolkit/toolkit.mozbuild' % CONFIG['mozreltopsrcdir'])
-
 if CONFIG['MOZ_EXTENSIONS']:
-    DIRS += ['/%s/extensions' % CONFIG['mozreltopsrcdir']]
+    DIRS += ['/extensions']
 
-DIRS += ['/%s' % CONFIG['MOZ_BRANDING_DIRECTORY']]
-
+if CONFIG['EXTERNAL_SOURCE_DIR']:
+    EXTERNAL_SOURCE_DIRS += ['/{}'.format(CONFIG['MOZ_BRANDING_DIRECTORY'])]
+else:
+    DIRS += ['/{}'.format(CONFIG['MOZ_BRANDING_DIRECTORY'])]
 
 if CONFIG['MOZ_CALENDAR']:
-    DIRS += [
-        '/%s/calendar/lightning' % CONFIG['commreltopsrcdir'],
-        '/%s/calendar/timezones' % CONFIG['commreltopsrcdir']
-    ]
+    AddCommDirs([
+        '/calendar/lightning',
+        '/calendar/timezones',
+    ])
 
-DIRS += [
-    '/%s/chat' % CONFIG['commreltopsrcdir'],
-    '/%s/editor/ui' % CONFIG['commreltopsrcdir'],
-    '/%s/mail' % CONFIG['commreltopsrcdir'],
-]
+AddCommDirs([
+    '/chat',
+    '/editor/ui',
+    '/mail',
+    '/mail/components',
+])

--- a/mailnews/mailnews.mozbuild
+++ b/mailnews/mailnews.mozbuild
@@ -1,16 +1,23 @@
 # vim: set filetype=python:
 # 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/.
 
-if CONFIG['MOZ_LDAP_XPCOM']:
-    DIRS += [
-        '/%s/ldap' % CONFIG['commreltopsrcdir'],
-        '/%s/ldap/xpcom' % CONFIG['commreltopsrcdir'],
-    ]
+@template
+def AddCommDirs(dirs):
+    if CONFIG['EXTERNAL_SOURCE_DIR']:
+        EXTERNAL_SOURCE_DIRS += dirs
+    else:
+        DIRS += ['/comm/{}'.format(dir) for dir in dirs]
+
 
-DIRS += [
-    '/%s/mailnews' % CONFIG['commreltopsrcdir'],
-    '/%s/db' % CONFIG['commreltopsrcdir'],
-]
+if CONFIG['MOZ_LDAP_XPCOM']:
+    AddCommDirs([
+        '/ldap',
+        '/ldap/xpcom',
+    ])
 
+AddCommDirs([
+    '/mailnews',
+    '/db',
+])

--- a/suite/app.mozbuild
+++ b/suite/app.mozbuild
@@ -1,25 +1,26 @@
 # vim: set filetype=python:
 # 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/.
 
-include('/%s/mailnews/mailnews.mozbuild' % CONFIG['commreltopsrcdir'])
-
 include('/toolkit/toolkit.mozbuild')
+include('../mailnews/mailnews.mozbuild')
 
 if CONFIG['MOZ_EXTENSIONS']:
     DIRS += ['/extensions']
 
-
-DIRS += ['/%s' % CONFIG['MOZ_BRANDING_DIRECTORY']]
+if CONFIG['EXTERNAL_SOURCE_DIR']:
+    EXTERNAL_SOURCE_DIRS += ['/{}'.format(CONFIG['MOZ_BRANDING_DIRECTORY'])]
+else:
+    DIRS += ['/{}'.format(CONFIG['MOZ_BRANDING_DIRECTORY'])]
 
 if CONFIG['MOZ_CALENDAR']:
-    DIRS += [
-        '/%s/calendar/lightning' % CONFIG['commreltopsrcdir'],
-        '/%s/calendar/timezones' % CONFIG['commreltopsrcdir']
-    ]
+    AddCommDirs([
+        '/calendar/lightning',
+        '/calendar/timezones',
+    ])
 
-DIRS += [
-    '/%s/editor/ui' % CONFIG['commreltopsrcdir'],
-    '/%s/suite' % CONFIG['commreltopsrcdir'],
-]
+AddCommDirs([
+    '/editor/ui',
+    '/suite',
+])