How to change default email program?

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http://forums.opensuse.org – I use KDE but use Thunderbird for email and Firefox for web. Both are working fine. However, when I click a mailto link in Firefox, the KDE mail ap starts up and I'd like it to go to TBird. I look in Firefox preferences, and under Applications > mailto, KDE is the listed application, but when I click "other" TBird is not listed in the "known applications" and I can't figure out how to select it. Why isn't it "known"? Tbird is properly installed and running fine. Sorry if this is a dumb newbie question. George in Canada (Distributions)