>>22170119I have 709 tabs open on my firefox and it's only using half as much memory as yours.
It seems to be caused by mainly two things: having more tabs loaded (ctrl-tabbing through a bunch of them increased my firexox's memory usage by 500mb) and the tabs actually doing something (unpausing my music on youtube added 70mb memory usage).
Just the sheer amount of unloaded tabs probably doesn't do much by itself.