The (current) state of Firefox Nightly and old extensions
Back in January in my entry on how ready my Firefox extensions are for Firefox Electrolysis, I said that Firefox's release calendar suggested that Firefox's development version (aka 'Nightly') would stop supporting old non-Electrolysis extensions some time around June or July. It's now mid June and some things have happened, but I'm not sure where Mozilla's timeline is on this. So here is what I know.
At the start of May, Firefox Nightly landed bug 1352204, which is
about disabling a lot of older extensions on Nightly. Mozilla has
an information page about this in their wiki, and various news
outlets noticed and reported on this change shortly after it went
live, which means I'm late to the party here. As the Mozilla page covers, you can fix
this by setting the
true. I've done this
ever since I found the option and everything appears to still work
fine in the current Nightly.
(I had some weird things happen with Youtube that caused me to not update my Firefox build for a month or so because I didn't want to deal with tracking the issue down, but when I started to test more extensively they went away. Problems that vanish on their own can be the best problems.)
This change itself doesn't seem to be how Mozilla intends to turn off old extensions, theoretically in Firefox 57. That seems to be bug 1336576, expanded in a Mozilla wiki entry. Based on the Mozilla wiki entry, it appears that Firefox's development code base (and thus Nightly) will continue to allow you to load old extensions even after Firefox 57 is released provided that you flip a magic preference. Firefox 57 itself will not allow you to do so; the preference will apparently do nothing.
As long as Mozilla has legacy extensions that they care about, I believe that the actual code to load and operate such extensions will be present and working in the Firefox code base; this is the 'signed by Mozilla internally' case in their compatibility table. This implies that even if Mozilla disables the preference in the development version, you can force-override this with a code change if you build your own Firefox (which is what I do). You may not be able to turn Electrolysis on if you have such old legacy extensions, but presumably your addons are more important than Electrolysis (this is certainly the case for me).
All of this makes me much happier about the state of my personal Firefox than I used to be, because it looks like the point where many of my current extensions will fall over is much further away than I thought it was. Far from being this summer, it may be next summer, or evn further away than that, and perhaps by then the release of Firefox 57+ will have caused more of the addons that I care about to be updated.
(However, not all of the omens for updated addons are good. For example, Self-Destructing Cookies now explicitly marks itself as incompatible with Electrolysis because apparently addon can't monitor sites' LocalStorage usage in e10s. This suggests that there are important gaps in what addons can now do, gaps that Mozilla may or may not close over time. At least this particular case is a known issue, though; see bugs 1333050, 1329745, and 1340511 (via the addons page for Cookie Autodelete, which I was recently pointed at by a helpful reader of Wandering Thoughts).)