I've never evangalised so hard about a piece of software, and every Linux and Apple user owe it a debt of gratitude, in as much that it was a good cross platform browser.
Performance wise, some upgrades have been for the better some for the worst. But don't get too sentimental about past incarnations. I remember there being memory issues in the 2.x line. I saw the browser eating over a half gig of ram once - and I nearly fell off my chair.
Those that have dozens of tabs open - I do think you need to ask yourself why? It just slows your computer down. I think we use them as replacement for bookmarks, which says something about the browser UI.
I've only recently left 3.6 to try out TenFourFox on my power pc - which at first felt a million times better, but after my initial excitement, I noticed it's a bit of a cpu hog. Idling with Gmail open, it seems to be quite greedy. Which suggests to me that there is an inherent problem with the browser - imagine if I had a handful of web apps open.
To say Firefox is broken is a bit strong. It would be better to say it has it's faults and could be faster. If it's DB is a bottleneck - could it be swapped for something else?
I do however think the browser UI is in serious need of some love, and could do with some innovation. It's barely changed. For example tab management is dire.