[notafox]

There's also Violentmonkey [1][2] (which is said to be more lightweight) and Tampermonkey.

Tampermonkey has analytics enabled, and is closed source.

Here's a comprehensive summary on all three. [3]

[1] https://addons.mozilla.org/en-US/firefox/addon/violentmonkey

[2] https://violentmonkey.github.io/

[3] https://news.ycombinator.com/item?id=26057106