So I had an image that was not loading, and could not figure out why, in Firefox 3. Turns out it was a plugin, mos tlikely adBlock plus. When all plugins were disabled, the image loaded fine. Here were the URLs which the plugins blocked, and the one it let by. Looks like the plugin blocks certain URLs based on specific words.
Always worked:
<img src="http://mydomain.com/comm/cmainpromo.jpg" alt="test">
Only worked with plugins disabled:
<img src="http://mydomain.com/comm/ads/siteads/cmainpromo.jpg" alt="test">
<img src="http://mydomain.com/comm/ads/cmainpromo.jpg" alt="test">