I'm not too sure why you might want to use mozilla mail, but open the links in firefox - afterall, mozilla is more "fully featured", isn't it?
If it was thunderbird (the matching mail client to firefox), then you just search for your thunderbird files, you'd want the prefs.js file and then add something like this
my prefs.js file is in the file:/home/john/.thunderbird/mb4eu9d7.default directory - you could always see if mozilla mail has something similar? afterall, thunderbird and mozilla mail are both mozilla app's.
p.s. Oh and if it is similar/the same, you'd just change "opera" to "firefox" and it should work!