Eyeballing the firefox sea, it's like... 3/5ths music/gaming related youtube vids, 2/5ths reading/reading adjacent (manga, webcomics), and then maybe 1/5th life-maintenance junk (bills, etc.) and miscellaneous (i.e. b12 and such). The tablet's opera ocean (can't even estimate, but it's somewhere well over 300) is probably 9/10ths reading, 1/10th stuff-not-that. I close tabs pretty regularly, but, like. I read voraciously, on the tune of a trade paperback's worth daily as a baseline, slow-day-busy-with-other-things level, and I'll bounce between several stories at once, follow recs several layers down, open stuff up for later perusal, etc. The behavior translates to wandering around let's play stuff and music, too.
It's definitely not common browsing behavior, but it's genuinely not difficult to build up like this if you're just willing to roam a bit and don't close tabs until you're proper done with it and don't intend to come back or branch off.
I'm not nearly as extensive at work, but I still end up with a few dozen tabs pretty regularly (on top of a bit under a dozen baseline, pre-opened to sites I use regularly, there's like six on chrome and then another four or five on firefox, there), especially when I'm dealing with batches of books that might have me looking at several dozen individual items more or less at the same time or in rapid sequential order. It's just a matter of opening tabs as needed and leaving them up if there's a possibility you might come back to it in the near future for some reason. Saves you from having to navigate back or whatever if the page you'd need to use is already up.
A fair amount of it all is just 'cause I don't like using bookmarks. I've found I'll regularly come back to open tabs, sooner or later, but bookmarked stuff just never gets seen again. Something about it functionally being stuck in a filing cabinet as opposed to still sitting on your desk means it ne'er gets touched again, y'know?