LastPass and uBlock Origin have Chrome versions so I have some degree of confidence they will be ported over.
FireGestures is absolutely essential to me, and no confidence there. Once Mozilla stops supporting XUL extensions I guess I'll use a Firefox fork or switch to Vivaldi. After... geez... 13 years of using Firefox. They finally forced me away.
Hopefully Vivaldi supports customizable mouse chord gestures by then.
Foxy Gestures in particular claims to be deeply inspired by FireGestures: https://addons.mozilla.org/firefox/addon/foxy-gestures/
But thanks for the link to Foxy Gestures-- I'll give it a shot!
Edit: Tried out Foxy Gestures. Not usable. First, no support for mouse button chording. Second, every single gesture pops up the right-click menu. Third, gestures don't work maybe 75% of the time-- difficult to tell if that's due to the right click menu popping up or if it's just plain broken.
Seems like it's this bug at fault. Mouse gesture Webextensions are broken on MacOS and Linux.
(Though it's probably worth letting the Foxy Gestures author know what bits seem to be missing as a FireGestures user.)
I did search and it looks like HTTPS Everywhere, LastPass, and RescueTime aren't compatible. Privacy Badger is compatible, which may satisfy people looking for an ad blocker.