I'm using Firefox 5 although this was also happening on version 4, when I have a lot of tabs open or if I run it for too long navigating to a web page will stop loading until I move my mouse cursor over the tab bar. I used to have a problem where I just needed to move the mouse to get the page to finish loading but oddly even that doesn't really work now, it has to be moved over the tab bar at the top of the browser window. Does anyone else have this problem?

Try disabling ALL your add-ons and see if this clears up. If it does, enable the add-ons one-by-one, restarting Firefox and testing in between. Enable the add-ons in the order of your favorite ones first until you figure out which (in any) are causing it to lock up.