Can you be more specific about the kinds of problems you are seeing? Can you reproduce these problems in safe mode? You should file a bug at http://bugzilla.mozilla.org/ and put [MemShrink] in the whiteboard.

Check what extensions you are using. Firefox by itself uses much less memory and is very fast now.

I cleaned up a lot of my extensions, and the memory usage went way down. I still get leaks where having a dozen or so tabs open will, over time, use up much of my free memory. This happens in Chrome as well, so I just keep that in mind and occasionally restart the browser.