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.
Mull is really great, +1