I have not had much luck with Firefox on Ubuntu lately. I booted up into Ubuntu earlier today and ran the software updates which included an upgraded to Firefox 188.8.131.52 (release not nightly). Now, I am back to the point where Firefox just freezes after a couple minutes. Not sure what is causing this, last time it was due to the Kernel update around Memorial Day, but that was fixed with another Kernel update and going to 184.108.40.206. I suspect it may be something gone afoul in my profile, so next time I will start with a fresh profile.
While I was in Ubuntu I did have a chance to take a brief look at the Epiphany Browser, a Gecko based browser for the GNOME desktop. The Epiphany Extensions Package is a collection of extensions which also includes the popular GreaseMonkey extension. However according to Wikipeida:
Greasemonkey is available for Firefox, Flock and Epiphany. The Epiphany Greasemonkey extension is part of the Epiphany-extensions package. However, this extension is not fully compatible as of release 2.15.1, since some Greasemonkey API functions (e.g. GM_getValue) are unsupported.
It does not appear this has been resolved in the latest version 2.18.1 as I still could not get Epiphany to install a GreaseMonkey script.