This seems extremely weird, never seen anything like that (I tend to have >100 tabs open). But of course "it doesn't happen for others" doesn't help you if somehow on your machine it does.
It's definitely strange. And I do at times have 50+ tabs open on Firefox, so I'm not saying it's a constant issue, but I do see it overload my system from time to time. When it does, I close a few tabs, restart firefox, and go on my merry way.