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/

I wrote a script and it just f*cking won't work.
I scavenged the whole help page and nothing f*cking helped.
I copied the whole script and threw it to the developer console. It f*cking works.

This add-on is a joke.