This problem affects TB 15 and TB 16. Daily does not appear to be affected.
So it looks like https://hg.mozilla.org/releases/comm-beta/rev/7b51c0455855 (https://hg.mozilla.org/comm-central/rev/f0160c7f11c9) actually depending on a previous landing (https://hg.mozilla.org/comm-central/rev/f03b55eda4ce), which did NOT land on comm-beta. I'm spinning up a patch now.
Created attachment 648338 [details] [diff] [review] Patch v1 (untested) This is untested: I don't have comm-central/beta installed or the source available or a way to build it currently. The fix is fairly trivial though.
Comment on attachment 648338 [details] [diff] [review] Patch v1 (untested) I can confirm that this fixes the bug. Your patch, however, has Windows style line endings. I'll upload a fixed patch.
Created attachment 648383 [details] [diff] [review] Patch v2 (by clokep) Patch with better line endings.
comm-aurora: https://hg.mozilla.org/releases/comm-aurora/rev/ccec873292af comm-beta: https://hg.mozilla.org/releases/comm-beta/rev/d30295ff2691