For me, tab location is spacial in my mind, and so many years of having them horizontal makes it hard to switch. Somewhere between 25 and 40 I start having trouble finding things at which point I just close them all and start over. I do use the tab drop-down in Firefox, but mostly to more quickly find things I want to close. Also completion in the url bar to find specific open tabs is quite useful.
This is a valid reason to switch back, but I'd also like to call out that UX designers need to do more for users with limited horizontal space. Many users rely on split-screen mode to see two windows simultaneously, or maybe three windows simultaneously if they've got a widescreen 21:9 aspect ratio monitor.