You can look at the network.protocol-handler.external.mailto and other mailto prefs on the about:config page.
You can open about: pages via the location bar like you open a website. Filter: mailto

If network.protocol-handler.external.mailto prefs are user set (bold) then right-click and Reset them to the default value.
You can also try to change the network.protocol-handler.warn-external.mailto pref to true.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.

Following your instructions I was able to reset: network.protocol handler.external.mailto prefs to default...it was set (bold) it's value reads True. However; I also reset the network.protocol-handler.warn-external.mailto prefs to default but the value is set to "False". When I right click I'm not sure if one of the options will change the value and if so which one? Does it matter that the value is False? Also; the network.protocol-handler.expose.mailto...is set to default but again value is "False". Am I good to go or may something not work wright along the way or create a security risk? I went back to Tools, Options, applications again and tried all three email programs mentioned in my question and which ever one I set is the one that opened. Thank you for your help...