Firefox localizations should contain all the strings to use them for builds on - central - aurora - beta - release - esr (possibly two) This also applies to Firefox for Aurora, Thunderbird, SeaMonkey.
To make localizations be applicable to all channels, we need all strings we have in each channel in a single repository per locale. The least tricky way to do this right now seems to be to create a single repository with all the en-US strings, and then expose that to localizers. This bug will track the individual work items. The next ESR is 52, so it'd be good to have this working by the time we merge to aurora.