diff --git a/build/mobile/devicemanagerADB.py b/build/mobile/devicemanagerADB.py
index 1ae1a527aef9..e6edd72a707b 100644
--- a/build/mobile/devicemanagerADB.py
+++ b/build/mobile/devicemanagerADB.py
@@ -12,7 +12,7 @@ import tempfile
 class DeviceManagerADB(DeviceManager):
 
   def __init__(self, host=None, port=20701, retrylimit=5, packageName='fennec',
-               adbPath='adb', deviceSerial=None):
+               adbPath='adb', deviceSerial=None, deviceRoot=None):
     self.host = host
     self.port = port
     self.retrylimit = retrylimit
@@ -24,7 +24,7 @@ class DeviceManagerADB(DeviceManager):
     self.useZip = False
     self.packageName = None
     self.tempDir = None
-    self.deviceRoot = None
+    self.deviceRoot = deviceRoot
 
     # the path to adb, or 'adb' to assume that it's on the PATH
     self.adbPath = adbPath
@@ -537,6 +537,13 @@ class DeviceManagerADB(DeviceManager):
 
   # Internal method to setup the device root and cache its value
   def setupDeviceRoot(self):
+    # if self.deviceRoot is already set, create it if necessary, and use it
+    if self.deviceRoot:
+      if not self.dirExists(self.deviceRoot):
+        if not self.mkDir(self.deviceRoot):
+          raise DMError("Unable to create device root %s" % self.deviceRoot)
+      return
+
     # /mnt/sdcard/tests is preferred to /data/local/tests, but this can be
     # over-ridden by creating /data/local/tests
     testRoot = "/data/local/tests"
diff --git a/build/mobile/devicemanagerSUT.py b/build/mobile/devicemanagerSUT.py
index 70d7b2b1b1c3..1204e50019a5 100644
--- a/build/mobile/devicemanagerSUT.py
+++ b/build/mobile/devicemanagerSUT.py
@@ -44,12 +44,13 @@ class DeviceManagerSUT(DeviceManager):
   # The error would be set where appropriate--so sendCMD() could set socket errors,
   # pushFile() and other file-related commands could set filesystem errors, etc.
 
-  def __init__(self, host, port = 20701, retrylimit = 5):
+  def __init__(self, host, port = 20701, retrylimit = 5, deviceRoot = None):
     self.host = host
     self.port = port
     self.retrylimit = retrylimit
     self.retries = 0
     self._sock = None
+    self.deviceRoot = deviceRoot
     if self.getDeviceRoot() == None:
         raise BaseException("Failed to connect to SUT Agent and retrieve the device root.")
 
@@ -838,18 +839,22 @@ class DeviceManagerSUT(DeviceManager):
   #  success: path for device root
   #  failure: None
   def getDeviceRoot(self):
-    try:
-      data = self.runCmds([{ 'cmd': 'testroot' }])
-    except:
-      return None
+    if self.deviceRoot:
+      deviceRoot = self.deviceRoot
+    else:
+      try:
+        data = self.runCmds([{ 'cmd': 'testroot' }])
+      except:
+        return None
 
-    deviceRoot = data.strip() + '/tests'
+      deviceRoot = data.strip() + '/tests'
 
     if (not self.dirExists(deviceRoot)):
       if (self.mkDir(deviceRoot) == None):
         return None
 
-    return deviceRoot
+    self.deviceRoot = deviceRoot
+    return self.deviceRoot
 
   def getAppRoot(self, packageName):
     try:
diff --git a/testing/mochitest/runtestsb2g.py b/testing/mochitest/runtestsb2g.py
index d52595b29c04..bdcb5a37cdea 100644
--- a/testing/mochitest/runtestsb2g.py
+++ b/testing/mochitest/runtestsb2g.py
@@ -183,6 +183,7 @@ class B2GMochitest(Mochitest):
     _automation = None
     _dm = None
     localProfile = None
+    testDir = '/data/local/tests'
 
     def __init__(self, automation, devmgr, options):
         self._automation = automation
@@ -193,7 +194,6 @@ class B2GMochitest(Mochitest):
         self._automation.setRemoteProfile(self.remoteProfile)
         self.remoteLog = options.remoteLogFile
         self.userJS = '/data/local/user.js'
-        self.testDir = '/data/local/tests'
         self.remoteMozillaPath = '/data/b2g/mozilla'
         self.remoteProfilesIniPath = os.path.join(self.remoteMozillaPath, 'profiles.ini')
         self.originalProfilesIni = None
@@ -405,7 +405,8 @@ def main():
     auto.marionette = marionette
 
     # create the DeviceManager
-    kwargs = {'adbPath': options.adbPath}
+    kwargs = {'adbPath': options.adbPath,
+              'deviceRoot': B2GMochitest.testDir}
     if options.deviceIP:
         kwargs.update({'host': options.deviceIP,
                        'port': options.devicePort})
@@ -421,10 +422,6 @@ def main():
 
     mochitest = B2GMochitest(auto, dm, options)
 
-    # Create /data/local/tests, to force its use by DeviceManagerADB;
-    # B2G won't run correctly with the profile installed to /mnt/sdcard.
-    dm.mkDirs(mochitest.testDir)
-
     options = parser.verifyOptions(options, mochitest)
     if (options == None):
         sys.exit(1)