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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