A few people use Tree-Style Tabs, which organize them as a tree structure, similar to directories. It encourages using tabs instead of bookmarks, which have a terrible UX (in all browsers, not picking on Firefox here).
Tree Style Tabs is the primary reason why I stick with Firefox. Without it, I wouldn't even know where to begin managing the relevant sites/info I encounter when reading up on diverging lines-of-thought.
For me, it is /the/ absolute killer feature for browsers.
Not sure how he does it, but I don't manage those. I just don't close tabs (I usually forget to do it :), so after week or two I end up with 200-300 tabs (my record is 900).
I tried to use chrome once and it ate all my ram, and they were saying it uses less ram then firefox back then.