Check the Mailto application in Firefox > Preferences > Applications
Make sure that it isn't set to the Firefox application.

See also:

• Firefox repeatedly opens empty tabs or windows after you click on a link

Dear cor-el thanks it appears to have fixed the problem. Thanks very much for the quick reply!!! Your instructions were very clear and helpful!!!!

You're welcome

Firefox > Preferences > Applications doesn't exist

Are you using a Mac or are you on another platform?

On other platforms use:

• Mac: Firefox > Preferences
• Linux: Edit > Preferences
• Windows: Firefox > Options

Press F10 if the menu bar is hidden on Windows and Linux or use "Firefox > Options/Preferences"

• http://kb.mozillazine.org/File_types_and_download_actions
• https://support.mozilla.org/kb/Managing+file+types