--- a/testing/mochitest/runtestsb2g.py
+++ b/testing/mochitest/runtestsb2g.py
@@ -339,19 +339,20 @@ def run_desktop_mochitests(parser, optio
     kwargs = {}
     if options.marionette:
         host, port = options.marionette.split(':')
         kwargs['host'] = host
         kwargs['port'] = int(port)
     marionette = Marionette.getMarionetteOrExit(**kwargs)
     mochitest = B2GDesktopMochitest(marionette, options.profile_data_dir)
 
-    # b2g desktop builds don't always have a b2g-bin file
-    if options.app[-4:] == '-bin':
-        options.app = options.app[:-4]
+    # add a -bin suffix if b2g-bin exists, but just b2g was specified
+    if options.app[-4:] != '-bin':
+        if os.path.isfile("%s-bin" % options.app):
+            options.app = "%s-bin" % options.app
 
     options = MochitestOptions.verifyOptions(parser, options, mochitest)
     if options == None:
         sys.exit(1)
 
     if options.desktop and not options.profile:
         raise Exception("must specify --profile when specifying --desktop")