Try clearing your cache and cookies.

1. Open the Tools menu, then select Clear Recent History... .
2. In the Time Range to clear: drop-down, select Everything.
3. Click the arrow next to Details to display the list of items that can be cleared.
4. Select both Cookies and Cache.
5. Click Clear Now .

It is possible that you clicked the 'Block Images' item in the right click context menu while trying to save an image. Check the image exceptions: Tools > Options > Content: Load Images: Exceptions - See Settings for web content, pop-ups, fonts, and languages

A way to see which images are blocked is to click the favicon (Site Identification icon) on the left side of the location bar. A click on the "More Information" button will open the Security tab of the "Page Info" window (also accessible via "Tools > Page Info"). Go to the Media tab of that "Page Info" window. Select the first image and scroll down though 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.

See also Fix problems that cause images to not show and http://kb.mozillazine.org/Images_or_animations_do_not_load