Firefox opens evolution instead of thunderbird for mailto links, even after suggested config changes

[David M. Karr]

This probably isn't an Ubuntu configuration issue, but more likely a 
Firefox configuration problem, but it is related to use on Ubuntu, and 
I'm not having much luck getting answers from Mozilla support.

I use Thunderbird instead of Evolution for email. When I follow a 
"mailto" link in Firefox, it opens an Evolution window.

I've followed advice on the internet about adding a line to the Firefox 
"prefs.js" to fix this:

user_pref("network.protocol-handler.app.mailto","/usr/bin/thunderbird");

Unfortunately, this has no effect (I also wonder how it would know to 
add "--compose" on that command line). After starting Firefox, I check 
"about:config", and it shows this setting, but "mailto" links still open 
evolution windows.

One link that describes how to resolve this, 
https://support.mozilla.com/en-US/kb/Changing+the+e-mail+program+used+by+Firefox 
, suggests that I edit the action for the "mailto" content type, and 
make it execute Thunderbird instead.  As a matter of fact, that's 
exactly how it's configured (probably because of my prefs.js change).  
Unfortunately, this has no effect.  My simple test case is entering 
"mailto:joe at joe.com" in the URL.  This brings up the "Evolution Setup 
Assistant" dialog.