Firefox 4 is known for becoming a real dog after you open a few tabs. This is caused by a default setting that greatly delays releasing unused memory that can eventually use all your memory and slow your browsing to a crawl. The Internet is filled with fixes that don't work. Here's one that does:
1. Open Firefox 4
2. Type "about:config" in the address bar.
2. Scroll down to "image.mem.min_discard_timeout_ms"
3. Change the default value from 120000 to 10000

Close that browser window, exit and relaunch Firefox 4 and your problems should be history.