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 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.