Application update relies on the incremental updater. App update has tests of its own that uses the incremental downloader, but it uses a mock implementation. It would be good to add new tests. There seems to be a manually run test here, but it would be good to have something that runs on each checkin. http://mxr.mozilla.org/mozilla-central/search?string=TestIncrementalDownload

Based on how my flame (with a July 31, 2014 build) is behaving, I have the suspicion that incremental downloads regressed at some point since bug 791829 was fixed. Perhaps that's since been fixed again, though.

Bulk change to priority: https://bugzilla.mozilla.org/show_bug.cgi?id=1399258