I use a slightly more compact layout that saves space and lets me see more tabs; put this in "Advanced->Extra style rules" in the addon preferences:
/* Compact tab layout */
:root { --tab-height: 20px !important; }
.tab { height: 20px !important; }
/* Shrink space between pinned tabs and tab bar, only when pins are present */
#tabbar[style*="margin"] { margin-top: 20px !important; }
I've used this extension for nearly a decade at this point, and I could never imagine switching back to a top tab bar. It scales very nicely to an arbitrary number of tabs, organizes them in a highly natural way, and gives you a bunch of new power-user ways to manage your tabs (close a whole tree, migrate subtrees to new windows, ...).
It's specifically an addon for folks who normally have dozens (or hundreds) of tabs open, and I can only imagine it's actively detrimental to someone who only has a handful of tabs at a time.
IMO you should just be thankful that your workflow/habits haven't develop in such a way that you benefit from the organizational power of tree style tabs ;-P
I'm a zillion tab person. But even aside from that, it improves the usability of the browser.
A problem with browser UI is that - given our monitor form factor - it makes text hard to read. Studies show that very wide columns of text are harder to read than something relatively narrower.
By changing the viewing pane of the browser, moving the tabs off the top and to the side, we mitigate that extra-wide shape somewhat, making the text more easily readable while giving ourselves a bit more vertical space.
You can make it look better by giving FF a dark theme. I agree it doesn't look great out of the box.
As for the use case, I don't think it's an understatement to say it changed how I browse the web. Before I discovered Tree Style Tabs, I was like you -- I only kept a few tabs open and was diligent about maintaining a coherent working set.
Now, I build hierarchical structures of nested tabs based on what I'm working on, and I easily reach the 60+ tab range on a daily basis. Instead of those tabs feeling like a burden, it's trivial to make use of all of them without feeling like anything is cluttered. I can maintain so much more organized information at my finger tips in Firefox than I ever could before.
I don't think it would work for me. My Firefox windows are 1366 pixels wide and screen height tall on a 4K screen. I don't have any sidebar space to waste. Maybe it's OK for people who maximize their browser windows on 16:9 screens.
I generally size my browser so that the viewport is roughly square - a wide squat browser usually leaves acres of margin, or on sites that don't have decent margin, enormously long lines that are all but impossible to read.
A square viewport on a widescreen monitor leaves loads of space for a sidebar.
This is how I do it, too. Given those 16:9 monitors, moving the tabs to the side is definitely an improvement in readability and efficient space usage.
I use a slightly more compact layout that saves space and lets me see more tabs; put this in "Advanced->Extra style rules" in the addon preferences:
I've used this extension for nearly a decade at this point, and I could never imagine switching back to a top tab bar. It scales very nicely to an arbitrary number of tabs, organizes them in a highly natural way, and gives you a bunch of new power-user ways to manage your tabs (close a whole tree, migrate subtrees to new windows, ...).