The Android service-based updater fails far too easily for a mobile device with spotty network connectivity. My phone has the tendency to drop the Wifi connection while downloading bigger files (don't ask! :-[), so the update notification just shows something along the lines of "Update download failed, tap to retry". When downloading the .apk using Google Chrome from nightly.mozilla.org it manages to recover and continue the download after these drops, and so should Firefox. I think the Download Manager of Firefox/Android has the same issue, btw. (Someone please make this block bug 786380.)
So I noticed that the Toolkit update service was extended in similar vein in bug 791829 for B2G. I suspect that doesn't help Firefox/Android? At least you can look at their strategy...
Depends on: 791829
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