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.
Not sure this is what you want, but in the Sidebery settings, you can change the “button width/height” in Styles Editor > Navigation Strip to change the size of the container buttons.
Edit: ah these are panels. I don’t really use containers, but you can override their indicator width with something like:
.Tab .ctx { width: 10px; }
in Sidebery’s style editor