funsize changes

gecko.diff

diff --git a/taskcluster/docker/funsize-update-generator/scripts/funsize.py b/taskcluster/docker/funsize-update-generator/scripts/funsize.py
--- a/taskcluster/docker/funsize-update-generator/scripts/funsize.py
+++ b/taskcluster/docker/funsize-update-generator/scripts/funsize.py
@@ -161,19 +161,21 @@ class WorkEnv(object):
     def download_buildsystem_bits(self, repo, revision):
         prefix = "{repo}/raw-file/{revision}/tools/update-packaging"
         prefix = prefix.format(repo=repo, revision=revision)
         for f in ("make_incremental_update.sh", "common.sh"):
             url = "{prefix}/{f}".format(prefix=prefix, f=f)
             download(url, dest=os.path.join(self.workdir, f), mode=0o755)

     def download_martools(self):
-        # TODO: check if the tools have to be branch specific
-        prefix = "https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/" \
-            "latest-mozilla-central/mar-tools/linux64"
+        prefix = os.environ.get('MAR_TOOLS_URL', '')
+        if len(prefix) == 0:
+            log.warning('Using mbsdiff and mar utilities from the tip of mozilla-central')
+            prefix = "https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/" \
+                "latest-mozilla-central/mar-tools/linux64"
         for f in ("mar", "mbsdiff"):
             url = "{prefix}/{f}".format(prefix=prefix, f=f)
             download(url, dest=os.path.join(self.workdir, f), mode=0o755)

     def cleanup(self):
         shutil.rmtree(self.workdir)

     @property