Very promising and already working...

Fast, light, intuitive, looks good. JS & CSS combined. Extremely responsive developer. Native userscripts API. Would recommend. One of my favorite extensions across any browser (Firefox, Safari, ug chromium, iCab, Orion)

+ fast & light
+ UI makes sense and doesn't require too many clicks for any given action (cough cough… ViolentMonkey)
+ responsive developer (responded within the day to multiple messages and willing to try to reproduce issue on own end)
+ well-done documentation, although it could be a little clearer for new users
+ runs both userCSS and userJS. I cannot understate how important this is. Having to run CSS through JS when using chromium is awful. Do not understand why other userscript managers (except, to my knowledge, Userscripts for Safari, another great manager) do not support this. Dealing with the lack of standardization of Stylish is not necessary, and you really shouldn't need a second extension for this anyway.
+ ability to set default templates
+ themes & extensive CSS & syntax options
+ native Firefox userscripts API

− some odd UI choices
• script-related buttons only visible when the "script" context is selected. This means you cannot directly create a new script when opening settings unless first you choose a script from the sidebar or the "Script & CSS" button in the top toolbar
• the location of the toggle button makes it look like you're enabling/disabling the individual script you are viewing, even if multiple are chosen
• other small confusing behaviors like this, almost all related to the choice to place almost all of the action buttons in the script context view. Not really confusing if actually working in the settings, but was confusing at first due to trying to open and create a script, but buttons not available. This is the main reason for 4 stars, but it is a single developer and I'm sure they would be open to feedback in this situation.
− slow and somewhat unsatisfying save animation, but it may be necessary, given I've had issues in the past when saving and continuing with other actions too quickly. Unsure if this is FireMonkey- or Firefox-related.
− Extension seemingly chooses at random whether or not a script is enabled when importing; although whatever logic is being used actually does seem to somehow choose accurately most of the time, and it's usually defaulting to enabled

Is it possible, by any chance, at some point, to get something like uBlock's procedural cosmetic filters for our UserCSS scripts? CSS is great, but it's a bit lacking in functionality, especially in regex support.
I'm only starting to learn JS, and I imagine it's gonna take a while before I'll be able to write the scripts I need.
Thanks for all your hard work!

...

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!).

Seems like we can't import scripts from zip
Right, unzip everything and import works, but slightly inconvenient, thank you for replying.