Try to delete the mimeTypes.rdf file in the Firefox Profile Folder.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

Did not appear to solve the problem. "Save image as..." still tries to tack an rb extension on everything. Tried three different sites.

Inspect the MIME database key with the registry editor (regedit.exe) and do a search for that MIME type (file extension) via Ctrl+F.
Be careful with editing the registry as there is no Undo possible: all changes are applied immediately.
You can export key(s) before making changes.
MIME Database : HKEY_CLASSES_ROOT\MIME\Database\Content Type

You can also check those file extension in the main HKEY_CLASSES_ROOT\ branch.

Found the problem. It was not in the registry. One of the Selenium IDE formatters (I suspect the Ruby one from the extension being forced) was interfering with the save image process. Issue can be closed.

Thank you for the help and suggestions.