I'm not sure that there will be many XUL extensions left maintained after a while -- since no major browser supports them and the fact that WebExtensions is a much easier framework for development. As a result, I don't see much of a future in Firefox pre-Quantum forks.

Yes, in time there won't be any XUL extenions. But what do they say we are all supposed do with that tiny nagging:'it worked yesterday, but not today' problem?

Firefox's Extended Support Release, which will keep getting updates for almost a year, certainly helps.

I've been on ESR because of vimperator/pentadactyl not being portable, plus some other extensions like FoxyProxy, Tree Style Tabs etc.

But today I discovered tridactyl works well enough on latest FF to suit my vim-ish needs, while the other two have been fairly completely ported.

Thanks for the tip about tridactyl, I've only seen Saka Key so far in this space.

Neat saka tridactyl and vim vixen several interesting choices

I did not like Saka Key (some weird behavior sometimes), but Vim Vixen comes close to VimFx.

Plus, there's already Pale Moon, which shares a lot of the same goals that this one seems to.