Mozilla, IE, Netscape, Image viewer, any image editor, some versions of Paint, and a host of other applications can be associated with JPG files for opening and viewing.
I have had problems with programs hi-jacking the JPG association, and had togo into the registry to change the association.
If you have an image editor, you should be able to go into settings or preferences, and set it to be the default application for JPG files.
Alternately, you can use an association manager to change what programs are the default application for which file types.
Check the links at www.nonags.com for free-ware association managers.
Chas