Firefox not displaying pages correctly

I'm new to Mac (as of Oct. 3rd), but I'm a long-time user of Firefox. I notice that when I have FF running for long periods of time, and I go to, for example, xbox.com to check out my friends list on the computer, often times the page shows up as plain text, no graphics or images or anything. Then I come here to create a post, and all of the buttons and formatting tools appear as plain text as well, instead of the images/buttons. I don't regularly visit a wide variety of web-sites, but the one i mentioned earlier seems to consistently trigger it. Usually the only way I can get pages to display correctly again (no matter where I try to go) is to completely quit Firefox and reopen it. I included a screen shot so you can see what it's coming up as, vs what it's supposed to look like. I've never had this problem with any websites as long as I've been using Firefox until just recently. Anyone know what's up with it?

http://img.photobucket.com/albums/v112/mach42006/Picture1.jpg
http://img.photobucket.com/albums/v112/mach42006/Picture2.jpg