Edit->Preferences->Content->Load Images -> [Exceptions] and unblock.

I think that is applicable on the old Firefox 3.5.* you are using.

You probably have blocked images from a domain like fbcdn.net that Facebook uses.

You can use these steps to check if images are blocked:

• Open the web page that has the images missing in a browser tab.
• Click the website favicon (How do I tell if my connection to a website is secure?) on the left end of the location bar.

• Click the "More Information" button to open the "Page Info" window with the Security tab selected (also accessible via "Tools > Page Info").
• Go to the Media tab of the "Tools > Page Info" window.

• Select the first image link and scroll down through the list with the Down arrow key.
• If an image in the list is grayed and there is a check-mark in the box "Block Images from..." then remove that mark to unblock the images from that domain.