diff --git a/build/automation.py.in b/build/automation.py.in
index 4cd5f6dfb6a..067e824eb8b 100644
--- a/build/automation.py.in
+++ b/build/automation.py.in
@@ -223,7 +223,7 @@ def readLocations(locationsPath = "server-locations.txt"):
   return locations
 
 
-def initializeProfile(profileDir):
+def initializeProfile(profileDir, extraPrefs = []):
   "Sets up the standard testing profile."
 
   # Start with a clean slate.
@@ -333,6 +333,14 @@ user_pref("camino.use_system_proxy_settings", false); // Camino-only, harmless t
 """ % {"pacURL": pacURL}
   prefs.append(part)
 
+  for v in extraPrefs:
+    thispref = v.split("=")
+    if len(thispref) < 2:
+      print "Error: syntax error in --extra-pref=" + v
+      sys.exit(1)
+    part = 'user_pref("%s", %s);\n' % (thispref[0], thispref[1])
+    prefs.append(part)
+
   # write the preferences
   prefsFile = open(profileDir + "/" + "user.js", "a")
   prefsFile.write("".join(prefs))
diff --git a/testing/mochitest/runtests.py.in b/testing/mochitest/runtests.py.in
index 725f3f4376a..cbe2383fec7 100644
--- a/testing/mochitest/runtests.py.in
+++ b/testing/mochitest/runtests.py.in
@@ -214,6 +214,12 @@ class MochitestOptions(optparse.OptionParser):
                            "environment")
     defaults["environment"] = []
     
+    self.add_option("--setpref",
+                    action = "append", type = "string",
+                    dest = "extraPrefs", metavar = "PREF=VALUE",
+                    help = "defines an extra user preference")
+    defaults["extraPrefs"] = []    
+    
     self.add_option("--browser-arg",
                     action = "append", type = "string",
                     dest = "browserArgs", metavar = "ARG",
@@ -426,7 +432,7 @@ Are you executing $objdir/_tests/testing/mochitest/runtests.py?"""
       sys.exit(1)
     browserEnv[v[:ix]] = v[ix + 1:]
 
-  automation.initializeProfile(PROFILE_DIRECTORY)
+  automation.initializeProfile(PROFILE_DIRECTORY, options.extraPrefs)
   manifest = addChromeToProfile(options)
   copyExtraFilesToProfile(options)
   server = MochitestServer(options)