Pentadactyl was all-but-abandoned for years before Vimperator was broken. It's been almost four years since it saw a release - which was for Firefox 24.0-30[0]. Its website still links at least three links to code.google.com on the front page! (I'm actually shocked if Pentadactyl has even been working for you until now, given that it was supposed to break a few release cycles ago - when e10s shipped - but maybe you just haven't updated Firefox in a while, or you manually disabled e10s.)
Vimperator - which is also incompatible with Firefox 55+ for the same reasons Pentadactyl is - at least has been receiving maintenance attention in the meantime[1], and also provides alternatives for use with Firefox 55+ (and 57+).
If you'd rather switch browsers entirely than use something like Vimium[2] on Firefox, go ahead, but it's hard to justify holding an entire browser back just to maintain compatibility with an extension that has been abandoned by its own maintainers. As I mentioned, I can't even reasonably expect them to hold Firefox back to maintain compatibility with the extensions I wrote and actively maintained. As a Firefox developer, I'm disappointed, but as a Firefox user, I'm more than happy enough with the changes to make up for it.
[0] http://5digits.org/pentadactyl/
[1] https://github.com/vimperator/vimperator-labs
[2] https://addons.mozilla.org/en-US/firefox/addon/vimium-ff/?sr...