Firefox keeps losing page-style for no reason?

[Daniel day]

I'm not too familiar with firefox to be able to really explain what it is doing, but firefox is regularly dropping the styles of pages I'm viewing - especially noticeable on websites such as Facebook. It will randomly lose all sense of page style - changing all the fonts to times new roman, making links unavailable and generally making life a pain.

It's definitely not a virus, as I'm using a brand new laptop without any firefox add ons or themes installed, and I have updated the software to the most recent version. I've found that by emptying my history/cookies cache and/or restarting firefox will allow websites to be viewed correctly again, but only for a short amount of time. This is getting pretty frustrating, is there any reason it keeps doing this? I have page coding set to Unicode/Western depending on what sites I'm visting, and I've made sure that the 'NO PAGE STYLE' tab isn't selected.

I've included a screen grab just incase my explanations don't make too much sense:

http://img14.imageshack.us/img14/764/firefoxmn.png