https://github.com/pentadactyl/pentadactyl
The development¹ has been continued by the community after Firefox removed support for XUL extensions.
¹Well, tbh basic maintenance is a more accurate term — the updates have been minor, and the patch that enables it in the latest version of Palemoon hasn’t been pushed to the addon store yet, you have to clone from master at the moment.
I've been trying out Tridactyl, and like it so far. I believe it's similar to Pentadactyl, though maybe missing some features.
http://bug.5digits.org/pentadactyl/