a good quote:
If you need compatibility with old scripts your best choice is Greasemonkey. If you need it for modern scripts or to write your own Violentmonkey is fine. Avoid Tampermonkey, they are not to be trusted (proprietary license and analytics)[1].

[1] https://www.reddit.com/r/firefox/comments/6hs59w/tampermonkey_has_google_analytics_enabled_by/

Works perfectly, and adding scripts is super simple. I highly recommend this over the more popular Tampermonkey, which is closed source, and Greasemonkey, which is now rarely updated and lacks features.