I just had to deal with this issue (photos showing correctly in IE and Chrome, but not Firefox). This is what I found:
If the URL to the photo uses backslashes it will not show in Firefox. Example: http:\\www.yoursite.com\photo.jpg
Make sure your URL is using forward slashes and the photos should show. Example: http://www.yoursite.com/photo.jpg
A URL with backslashes will correct itself when pasted into the Firefox address bar, but when such a URL is embedded it breaks. Not sure why this is an issue in Firefox, but I hope this solves your issue.