What Mac The Knife said!
But, in furtherance to that: there is no way to reclaim memory which was supposed to be deallocated by any given app aside from rebooting your system. The improperly deallocated RAM will remain in Limbo, so to speak.
Mac: later version of FF are indeed better at handling RAM BUT I still often see a huge memory print when opening and closing several tabs, over extended periods of usage. And closing FF does not always free up the RAM it was using. I think Mozilla as an organization needs to expend more resources in addresses FF's memory demands. (This is not the behavior I see when using Apple's Safari though!)