if you go to the "Firefox" menu and choose "Preferences..." then click on the "Content" icon and see if "Load images automatically" is checked or not. If it is not checked, check it, then click on the red circle to close that window. That may or may not help. Sometimes Firefox just kind of chokes and not all the images load, or pages don't load completely. And if you look at many pages in one session, that use the same images, and an image never loaded properly, it probably won't show up on any of those pages during that session. Another thing you might try is clearing out your cache. That will force the program to reload all images and that may fix the problem. Go to Firefox -> "Preferences..." again, but this time click on the "Privacy" icon, then click on the "Clear now" button on the lower right of that window, then just make sure only "Cache" is checked in the list that appears, and then click on "Clear Private Data Now." Then quit Firefox and restart it again. Then Firefox will have to reload all images and the ones that were missing should show up then. If this still doesn't fix your problem, then I would suggest maybe looking at these pages and possibly posting your question at a Firefox forum:
http://www.mozilla.com/en-US/firefox/tips/http://support.mozilla.com/en-US/kb/