Have you tried rebooting (restarting) your computer (not just the browser)? The problem you're describing has a nasty habit of popping up when you run out of memory (RAM) and have to rely on hard disc swapping. (Browsers don't help here; they're all pretty good at grabbing and not releasing memory just in case you may want to go back in history or re-open a closed tab. Adobe products are also "greedy" by default, which is good when you're using them - you don't have to wait for memory allocation every time you click on something - but not so good when you want to use another program.)