How can I open .jpeg image files automatically without having the pop-
up box always ask me "What should Firefox do with this file?"?

The "Open with PictureViewer.exe" and the "Do this automatically for
files like this from now on" boxes are checked. (That pop-up box
also says that the "Settings can be changed using the Application tab
in Firefox's Options" but I don't know what to do when I go there).