I've been a Tree Style Tab user for many years, but I have to confess I have a complicated relationship with this add-on. I've reached >600 tabs more than once. That's not only a feature.
I used to use Tab Mix Plus on Firefox. Having three rows of tabs and the ability to scroll them vertically for more tabs was the absolute killer feature for me. I loved Firefox for this.
Once Firefox moved to the per-process approach and removed the ability to hack the UI, I saw no more reason to stay on what was a terribly slow browser back then, compared to Chrome. Startup times of 10+ seconds and such shenanigans.
