At GitHub I already expressed my gratitude to the Dev @erosman, so my review below is for potential new FireMonkey' users.

It is an add-on for smart users!
FireMonkey easily replaces several other add-ons, using just 20% of the resources (RAM, CPU etc). The improvements in browser performance, security and privacy are visible.
In addition, FireMonkey is the only add-on (in its class) really Open Source, with the express commitment of the Dev not to invade privacy.
Another advantage is that FireMonkey uses the latest newest API.

Those users who don't care about performance, security, privacy, etc., and want lights, garlands, colors, glitter and fancy stuff ... FireMonkey may not be for them.

Some of the other similar add-ons have many automated functions, while in FireMonkey some are manual. At first I felt intimidated. But in less than 2 days of use, without problems I got used to everything. Today with few days of use, it seems like I have FireMonkey for years. I had practically no problems in the transition and migration of all my other add-ons/scripts (CSS & JS).
FireMonkey is a minimalist add-on, but totally powerful. Virtually any difficulty in the transition of other add-ons can be solved in FireMonkey (or at least almost solved). By the way, every excellent add-on requires a small investment of time at the beginning. But it's just a one-time-task/job.

The Dev (@erosman) is very responsive, and replies very promptly at GitHub. The few problems I found, the Dev solved them on the same day.

FireMonkey is not perfect, and it will undoubtedly have a path of evolution. But today, I can't imagine going back to use all the add-ons FireMonkey replaced.

Do you have doubts? Fears? Take a simple test!... it'll be difficult to repent.

Here I am giving 5 stars because it is the maximum, but FireMonkey deserves all the possible stars.

Attached a review from GHacks: https://www.ghacks.net/2019/09/10/firemonkey-uses-firefoxs-official-api-for-userscripts-and-userstyles/

Will be good, but right now doesn't compete with Tampermonkey. Needs easier method to install scripts than pasting in the code.

excellent. replaced both violerntmonkey and stylus for me, even imported my configs from those. only negative is the highly annoying animations/transitions in the settings of this extensions, but i can live with that as i dont plan on going into the settings very often

edit: i now found out that it takes like a second for it to change website fonts i have changed in fire monkey - this doesent occur with stylus. this is a little drawback. do you think it can be fixed? like i load a site, it loads with its original fonts, a second (or half a second) goes and it changes to the font i specified for this site in firemonkey

Very nice idea. But it doesn't allow you to install scripts or styles from the main repositories (like greasyfork.org or userstyles.org.) You would have to add them manually by copying and pasting the scripts' text I guess.

I tried importing my scripts from greasemonkey gsm folder but the one I tried (google hit hider) just returned an error.

Also don't know if it supports v4 greasemonkey scripts along with v3?

Looking forward to trying this out when these things are working.

Promising.

Jan 17, 2021: Please do keep up the good work. I am surprised to see so few users. Glad you finally gave up the buggy homegrown editor.
Mar 24, 2019: A promising start. I like the idea of combining user scripts and user css in one add-on. Developer is very responsive and promptly fixed some issues I reported. Please disregard one-star ratings. I inadvertently got dragged into this so I know what is going on with those. A silly attempt to hurt a fellow developer who is also volunteering to be a code reviewer for other add-ons.