Below are the steps to making Mozilla Thunderbird (or any mail client of your choosing) the default mail client for Firefox in Linux.

  1. enter about:config in the address field
  2. filter for mailto
  3. make sure that “network.protocol-handler.external.mailto” is set to true (double-click to change).
  4. Add a new preference:
    • right-click on the window
    • choose New, String
    • prefence name is network.protocol-handler.app.mailto
    • value is /path/to/thunderbird (if using another mail client other than thunderbird, enter the path to it here instead)