1) Launch Firefox and view the configuration strings using the URL

about:config

2) using the field at the top, search for mailto

3) If you do NOT have network.protocol-handler.app.mailto (normally
you don't), create it using these steps:
a) right-click
b) choose New --> String
c) new string: network.protocol-handler.app.mailto
d) new value: /usr/bin/mozilla-thunderbird

4) If you already see an entry for
network.protocol-handler.app.mailto, verify the path to the mail
client. Double click the string to set it if necessary. Also be sure
network.protocol-handler.external.mailto is set to true.