Please see Change the program used to open email links and report back if this resolved your isse.

Followed your suggestion. There were two choices 1) outlook Express 2) Windows Live Mail. Selected Outlook Express but did not work. Defaults back to "Use my Current Email Program"

The image you posted was from a Windows selection of email clients. Instead, see the article mentioned above for selecting an email client from within Firefox's preferences:

     Tools > Options > Applications

If you are not successful, please post back on the message Firefox gives you.

Screen print from Firefox >tools>Application. It shows Outlook Express for email selection. Still when I click to contact the person placed Advertisement in Craigslist, nothing happens.

If you change your mail helper application preference in Firefox to "Use Other..." you are presented with a prompt to select a helper application and can click "Browse..." and browse your computer to the program of choice. Even though you are already shown two mail programs, try re-selecting the one you want to use via the browse method and then try again to open a mailto link.

If this does not work, open your profile folder from the button presented in

     Help > Troubleshooting Information

and exit Firefox. With Firefox closed, delete the "mimeTypes.rdf" file from your profile folder, then restart Firefox and Firefox will re-build the helper applications file and you can try again to email someone from a link.

Deleted "mimetypes.rdf". Rebooted Firefox but no change.

Try to select "Always ask" this time in your

     Tools > Options > Applications

for mailto links. (see image link below for an example).

     https://support.mozilla.com/media/uploads/gallery/images/2011-06-09-07-50-41-a72089.png

Once you do this and exit your options panels, what happens when you click on a mailto link? Do you get an error message or any message at all?

Selected "Always Ask" as you suggested. Tried email link from Craigslist, nothing happened.