Right now if you make Firefox the default handler for mailto: (in a misguided attempt to make it work with webmail) or any file type that it can't actually handle, Firefox starts spawning blank tabs like crazy when you click on the appropriate link. This makes sense, of course, since Firefox gets a mail link, sends it to its default handler, and then it gets a mail link. This also would happen if you have it set to be the default handler for (say) irc or a document type like PDF. I found bug 501471 and bug 510009 but I'm sure there are others. I'm just filing a new bug to cover all cases, not just mailto.
There is also bug 570600
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