Tell me about it.
When I use Opera 10 this just doesn't happen.
I closed Firefox and my Memory usage dropped from 1.72GB to 330MB!
Now as I type this my Memory usage is 486MB. And thats after re-starting Firefox.
If I carry on browsing different pages, etc, the memory will increase every time. It seems to happen as I browse different pages.
Tell you what i'm gonna carry on till I hit 2.5GB and do a screenie. Not now though, it's late.
Just refreshed my Facebook home and it added 15MB to my RAM. In fact refreshing Facebook uses up more of my memory.