My Mac has been bugging me to let it install some updates for several days now (requiring a reboot), so since I was going out for the afternoon anyway, I let it do so.
I completely forgot that this would cause Firefox to update to version 57. Oops. (At work I both turned off automatic updates and did some prep work to update to add-ons that will continue to work in 57. I hadn't gotten around to updating add-ons at home, and I forgot that I hadn't turned off browser auto-updates.)
I've lost my Stylish CSS overrides. Some I shared between home and work (or between Firefox and Chrome at home), so those ones I have, but some sites I only visit at home so I didn't have those at work. I found some stuff about how to find them on a Windows machine, but the filenames mentioned there don't exist on my Mac.
For the most part I'm going to just live without them and migrate more of my browsing activity to Chrome. The main reason I limit Chrome is that the tabs display is totally unreadable if you have too many tabs, unlike Firefox which sets a minimum size and gives you scrolling and a drop-down menu to see all of them. I just found a Chrome extension that provides that drop-down menu, so I can at least find stuff, though I haven't yet found a way to get Chrome to stop trying to show all of them anyway.
I also found these instructions for doing some of the things that Classic Theme Restorer did.
I've updated my earlier post about Firefox 57 with other workarounds I've found. For userscripts, I installed TamperMonkey, which I'm already familiar with from Chrome. For both scripts and CSS, I decided that at home I'll just do all my Stack Exchange stuff in Chrome -- I mostly was anyway, and now that it'd be actual work to get those scripts and styles back, time to just commit to it. Firefox is now almost exclusively for blog-reading (mainly Dreamwidth and those few people still on LJ), and everything else I do in Chrome. (That's at home; at work I do a lot more in Firefox.) I tend to have a lot of DW tabs open, so keeping that activity in the browser that handles tabs better makes sense.