If I am opening and closing a lot of image-heavy tabs, after a while, my firefox instance is sucking up 800MB of system memory, and the ONLY way to free it is to restart firefox. This is because under some circumstances (Well, always, at least for me and many other users) it does NOT remove the uncompressed images from memory when a tab is closed. Firefox (on Windows) can and will suck up an infinite amount of memory.