I switched to Floorp two months ago since it has native vertical tabs (which I greatly prefer over Tree Style Tabs since its a little buggy) and it has been running fine, though earlier this week I got a bug where I set a hotkey and removing it didn't clear it (it still triggered), even after restarts. I tried switching to the tab to the left with CMD OPT Left arrow, and accidentally set a hotkey for toggle the synced tabs sidebar.
I use that hotkey all the time and it basically ruined my workflow, it takes much longer to switch tabs with the mouse. I just found a workaround where I run my Floorp profile in Floorp Daylight (the beta version of Floorp, equivalent to Firefox Beta) remove the hotkey there, then run regular Floorp again.
While I'm happy with Floorp's exclusive featrues, I really wish they were in upstream Firefox because this is a big downside of using a fork, it has its own bugs. And I have to wait for changes from newer Firefox versions to get merged.
There are significantly fewer Firefox-based browsers than there are Chromium-based ones, unfortunately. Out of the ones we do have:
Floorp has much like Vivaldi gone the proprietary source-available route, so you couldn't pay me to use it.
Pale Moon is easily the most involved of the Firefox forks, being a fork of a much older version of Firefox, but I wouldn't generally recommend it for security reasons. It does have its uses, though. Waterfox Classic is in a somewhat similar boat. Security-wise Pale Moon is definitely the better of the two because it uses its own fork of Gecko which is maintained about as well as you could reasonably expect given the manpower available to the project. Waterfox Classic meanwhile has kinda just been left to rot since most development is going to regular Waterfox nowadays, so it's not maintained nearly as well as Pale Moon and it's just been collecting CVEs. But for those same reasons if all you want is the ability to use legacy XUL extensions, then Waterfox Classic is gonna have better compatibility since it hasn't been modified nearly as heavily as Pale Moon.
LibreWolf is probably the most popular Firefox fork nowadays, but it isn't much more than a Firefox equivalent to Ungoogled Chromium. Waterfox goes a little further, but not by much. Both can be good choices, but personally I haven't had much reason to switch away from upstream Firefox. LibreWolf is tempting, but I can already disable pretty much all of the Firefox BS from about:config, so I don't see the point. It's pretty much just better defaults.