Firefox 57 and the state of old pre-WebExtensions addons
Back in June I wrote an entry about the current state of pre-WebExtensions addons and what I thought was the likely future of them. Based on Mozilla's wiki page on this, which said that legacy extensions could still be loaded on Nightly builds with a preference, I said I expected that they would keep working well into the future. As far as I can currently tell, I was wrong about that and my optimistic belief was completely off.
As of right now given the ongoing state of Firefox Nightly, it appears that Mozilla has abandoned practical compatibility with many old addons. With the preference set, Mozilla may allow Nightly to load them, but Mozilla won't necessarily keep them working; in fact, available evidence suggests that Mozilla has been cheerfully breaking old extension APIs left and right. The only old APIs that we can probably count on continuing to work are the APIs that Mozilla needs for their own use (the 'signed by Mozilla internally' status of legacy extensions in the wiki page), and it's clear that these APIs are not sufficient for many addons.
In particular, all of my addons remain in various states between broken and completely non-functional on Nightly. This situation has not been improving since the preference change landed; if anything they've been getting worse in more recent Nightly versions. Since this has persisted for weeks and weeks, I have to assume that Mozilla no longer cares about general legacy extensions on Nightly and they're either landing code improvements that have no support for them or are actively removing the code that supports legacy extension APIs. Or both. I can't blame Mozilla for this, since they've been saying for years now that they wanted to get rid of the old APIs and the old APIs were holding back Firefox development.
One implication of this is that Mozilla is now fairly strongly committed to their Firefox 57 extension plans, come what may. With legacy extensions broken in practice, Mozilla cannot simply re-flip the preference the other way to back out of the transition and return legacy extensions to operation. Nor do I think they have time to fix the code should they decide they want to. If I'm reading the Firefox release calendar correctly, we're about one or two weeks from Firefox Nightly transmuting into the Firefox 57 beta version, and then six weeks more until Firefox 57 is actually released.
The personal implication for me is that I've now reached the last Nightly version I can use, although it happened perhaps a month later than I thought it would way back when. Now that I look at the dates of things, I think my current Firefox 'Nightly' versions are actually before Firefox 56 branched off, so I should probably switch over to Firefox 56 and freeze on it. That way I'll at least get security fixes until November or so.