I'm always surprised when I find out more people don't know about this (or other vertical-tabs addons). I typically have dozens of tabs and keep them open for weeks at a time (ff is set to restore previous session. So most are usually unloaded, but I also have an addon to unload tabs at-will if it's ever a problem). Horizontal tabs feel so arcane after getting used to it, I'm surprised it's not a standard feature yet.

At that point how are they different from bookmarks? I suppose with tab tree every page is ‘bookmarked’ until you close it, or bookmark it for real.

Personally I save anything I actually want to refer back to as a PDF since the bookmarked web page will probably disappear.

Still the primary reason why I use Firefox. It allows me to have effectively an infinite number of tabs open, grouped, organized, and opened based on what I'm actually trying to get done now, or what I need to get back to later.

This is the main reason I use Firefox. Absolutely love Tree Style Tabs.