...

Nothing works properly both user-Scripts and user-Style are broken especially any .css seems to not being respected

I wanted to like FireMonkey, but unfortunately it seems to be incompatible with the vast majority of scripts I use with the Waze Map Editor downloaded from greasyfork.org. There are no JavaScript errors -- the scripts just do nothing. They are listed as active on the page, but something about them just doesn't work with this addon. So I'll have to switch to a different one.

The UI is much better and more intuitive than that of TemperMonkey.

I really like the distinction between CSS and JS modifications and that I don’t need to use “proprietary” GreaseMonkey functions like GM_addStyle (` /* the CSS goes here */ `).

FireMonkey also doesn’t exhibit the bug that TemperMonkey does — when I navigated the same portal over many pages, it seemed TM kept adding the same CSS script over and over, which, over time, slowed Firefox noticeably (after a while TM reported aplying about 70 scripts instead of 2!).

Perfect better than the alternatives.

It is indeed super lightweight. A nice advantage to being on Firefox, as it's not available elsewhere.