Since Greasemonkey is no longer being updated and /kbinStyles has been thriving, with people creating a lot of userscript for added functionality, I thought I should introduce Violentmonkey here. It is a really good opensource alternative for Tampermonkey and works well.
Supported browsers: Firefox, Chrome, Edge, Opera
You can check it out here: https://violentmonkey.github.io/
If you look for information about it (edit: of violentmonkey) years ago you will probably find negative reviews, as it had an ambiguous privacy policy that generated many suspicions and accusations; but they fixed it a long ago, it is an excellent addon.
If you are talking about tampermonkey, I am sure it is a good addon. But it is no longer opensource. This post was meant for people who prefer opensource over proprietary as much as possible.
Nope, my whole comment was about violentmonkey. Take a look at this.
Interesting, thanks.