My Ad-ons are Fire FTP, Skype Extension and Foxmarks (2.0.43).
Here are three sites that don't load properly in FireFox (188.8.131.52) :-
-Ebay logo not displaying only default text replacing the logo
-Menu background image top right hand corner not displaying only text
-Central image with the words "Be Inspired" not displayinghttp://maps.google.com.au
Map doesn't displayhttp://www.acquos.com/
Gif image linking to another page not displaying. The image should appear under the heading "Redispersable Polymers"
I don't think its the ad ons that are the problem as they don't cause a problem on another computer I use and are not the ones you mentioned.
Any light you can shed as to the cause would be appreciated greatly.