For Greasemonkey proper (which has always only been a Firefox extension) the big pain point was Mozilla's forced migration to new extension APIs (2015: https://blog.mozilla.org/addons/2015/08/21/the-future-of-dev... ). This required a major rewrite, taking over a year, and not to add new features but rather just to not bit rot away. Then what felt like right after that, they completely deprecated classic extensions, forcing only web extensions (2017: https://blog.mozilla.org/addons/2017/02/16/the-road-to-firef... ). This required an even more thorough rewrite again, and made it not difficult but actually impossible to keep all functionality.
Greasemonkey has been stable (not abandoned, but not worked on very much!) since then. No forced MV3 yet in Firefox.
Greasemonkey has been stable (not abandoned, but not worked on very much!) since then. No forced MV3 yet in Firefox.