I use Firefox on a 2011 MacBook Pro/Lion and I find that memory usage constantly increases, even when very little is going on (starts at about 200MB, soon reached 500MB). Because of this, I have switched to Chrome, whose memory stays about 200MB.
My fingers have gotten used to doing command-option-shift-click which opens a new tab AND switches to it.
Unless you have a leaking plugin, Firefox is likely doing that for performance. You have a lot more than 500MB RAM, right? Why not take advantage of it?
My fingers have gotten used to doing command-option-shift-click which opens a new tab AND switches to it.