mozex is what you need.
Follow the instructions, install it and restart mozilla.
Then you go into Edit/Preferences and you will see mozex under the Advanced category.
Select it.
You'll see "Intercept clicks" on the right, select "mailto" and in the "Commands" section add this line in the "Mailer" field.
Code:
/usr/bin/evolution mailto:%a?subject=%s
That should do it.