The best userscript manager for Firefox.

I used Greasemonkey forever, then switched to Tampermonkey after Firefox broke legacy support with its multi-process architecture. I still run both, but Violentmonkey outperforms its competitors in every way that matters.

No bloat, privacy-focused, actually optimised for Firefox and open-source. Tampermonkey being closed-source, and originally Chrome-centric (pre-Manifest V3). If you're using Firefox, use Violentmonkey.