My new worry about Firefox 56 and the addons that I care about
In light of Firefox 57 and the state of old addons, where my old addons don't work in Firefox Nightly (or didn't half a month ago), my plan is to use Firefox 56 for as long as possible. By 'as long as possible', I mean well beyond when Firefox 56 is officially supported; I hope to keep using it until it actively breaks or is too alarmingly insecure for me. Using an actual released version of Firefox instead of a development version is an odd feeling, but now that it's out, I'm seeing a trend in my addons that is giving me a new thing to worry about. Namely, a number of them are switching over to being WebExtensions addons as preparation for Firefox 57.
This switch would be nothing to worry about if Firefox's WebExtensions API was complete (and Firefox 56 had the complete API), but one of the problems with this whole switch is exactly that Firefox's WE API is far from complete. There are already any number of 'we need feature X' bug reports from various extensions, and I'm sure that more are coming as people attempt to migrate more things to WebExtensions so that they'll survive the transition to Firefox 57. Unless things go terribly wrong, this means that future versions of Firefox are going to pick up more and more WebExtensions APIs that aren't in Firefox 56, and addon authors are going to start using those APIs.
In short, it seems quite likely that sticking with Firefox 56 is also going to mean sticking with older versions of addons. Possibly I'll have to manually curate and freeze my addons to pre WebExtensions versions in order to get fully working addons (especially ones that don't leak memory, which is once again a problem with my Firefox setup, probably because recent updates to various addons have problems).
(Fortunately Mozilla generally or always makes older versions of addons installable on addons.mozilla.org if you poke the right things. But I'm not looking forward to bisecting addon versions to find ones that work right and don't leak memory too fast.)
The optimistic view of the current situation with Firefox addons is that the WebExtensions versions of popular addons are basically beta at this point because of relatively low use. With Firefox 56 released, people moving more aggressively to be ready for Firefox 57, and the (much) greater use of WebExtensions addons, the quality of WE addons will go up fairly rapidly even with the current Firefox WebExtensions APIs. This could give me stable and hopefully non-leaking addons before people move on and addons become incompatible with Firefox 56.