I do a lot of technology development and R&D. That requires lots of research and lots of learning, sometimes over a considerable period of time.
Tabs provide something that bookmarks don't - most notably, more context and state. Think about it, just the order and placement of tabs conveys additional dimensions of information beyond what a simple bookmark would. I would ideally have these scattered across a desk sized, high-res display, but since those don't exist, I use a bunch of tabs.
Flipping back and forth and finding what you want becomes very easy with powertools like TabMixPlus - I use three rows of tabs, set to scroll - with a scrollwheel mouse, this makes finding the tab I want very quick and easy. This keeps the tabs from squashing down to nothing, but I never give up more than three rows of precious vertical space on my damned mail-slot HD widescreen display. This becomes even more useful when you set default behavior to open a new tab after the present one rather than at the end - this creates a sort of virtual piling system for tabs, so things that are related wind up being fairly close to each other. With TMP's excellent replacement for the Firefox native session manager, and a bit of custom tab color coding, it's really not hard to know what's where. None of this is necessary, but it saves me hours (and more importantly, lots of frustration) each week. It *does* mean, though, that you begin to get paranoid about losing your tab sessions, since there's a non-trivial amount of state info in them...
Now, admittedly, I may be a pathological case when it come to tabs, but I currently have two computers in from of me (one "work" computer, the other my personal laptop, used for my day job as as well as personal business, consulting, and my own product development) - On each computer, my main browser, Firefox, is open with more than 137 tabs and 122 tabs, respectively. Those are largely different tabs, although some, like work gmail are open on both for convenience. This setup is not for everyone, but it works extremely well for me. I almost hate to admit it, but one of the computers also has Chrome up with another several dozen tabs...
By the way, I'm not against bookmarks - in fact, somewhere, I have an email from a member of the Mozilla development team thinking me for helping them debug very large hierarchical bookmarks files (many thousands of bookmarks, which was really a lot back in the early 2000's...) I use the crap out of bookmarks, too, but tabs are a great way to hold all the "working on it" stuff until it finally gets done - Just this weekend, I closed out a project and killed off a few dozen tabs related to it - turns out I only needed to keep tttttwo or three actual bookmarks, but the other tabs all had information that was valuable and relevant as I was researching...
Oh, and to address the comments of a poster below, I do all this on Windows 7, and it's quite acceptably stable for this sort of thing. I will say, though that only Firefox is really stable at more than about 100-120 tabs, and even it starts to run out of steam somewhere between 150 and 200 tabs. Chrome fails or flakes for me right around 100 tabs. Oh, well...