I’m trying to switch to Floorp right now from Firefox, where I have both the regular horizontal tabs, and a flat vertical list with the Tree Style Tab extension. I use the later a lot, and while I could keep this setup in Floorp, I like that the vertical tabs can be native instead of using TST. However, it just feels weird to not have horizontal tabs. I think I might miss seeing a bunch of my tab titles at a glance with it, but I have to use it more and see how I feel.
I’ve been using Firefox Vertical Tabs since 1 or 2 years ago and my experience has been satisfactory. Now, I’m not a vertical tabs power user or something like that, and if suddenly I cannot use them anymore I can go back to use horizontal tabs without much problem. At this point I’m just accustomed to them, but they’re not an integral part of my workflow.