Hmm, everything works fine now even though I didn't do anything (like cleaning cache). Weird.

Did you try enabling javascript on main page?
[CUT="Well, here are my Javascript settings in browser, everything should work fine, I guess"]:
browser.opaqueResponseBlocking.javascriptValidator true
browser.urlbar.filter.javascript true
devtools.debugger.features.javascript-tracing false ...

*There are no topics about bugs, so...*

I'm not actually sure it's a bug (I just might have disabled smth in preferences by accident), but Alphabet page in Firefox stopped properly functioning - I mean, nothing happens if you click on years, letters or languages. Disabling uBLock Origin for all ...