Mozilla Suite : Issues : Image settings
From MozillaZine Knowledge Base
Mozilla hijacked my picture settings
This was a reported bug for very long but should have been fixed in later versions. If Mozilla still hijacks settings for your pictures:
Open the Preferences dialog:
Edit | Preferences.
Open the Advanced | System category.
In the file association box, uncheck all checkboxes for images. Mozilla will revert them back to their previous settings.