Firefox – Open mailto:-links in GMail

I use GMail a lot. I also send a lot of emails, many of those originating from mailto:-links. Hence, it is practical to have mailto:-links open in GMail.
 
This is quickly and simply achieved by going to Tools, Options, Applications. In the search-field, simply enter mailto, then use the Action-field to select GMail, Yahoo Mail or something else, like so:
 

 
Some installations of Firefox do not have the GMail-alternative. This is simply fixed. Open a new tab, then enter about:config in the address-bar. Into the filter enter
gecko.handlerService.AllowRegisterFromDifferentHost, then doubleclick the value so that it is set to true. Then copy/paste the following piece of Javascript to your Addressbar: javascript:window.navigator.registerProtocolHandler("mailto", "https://mail.google.com/mail/?extsrc=mailto&url=%s","Gmail"). When asked whether you want to add GMail as an application, answer “yes”.