Redlemon

Aha, I think the bug is an edge case of:
Bug 246078 - URLs from other apps result in two Firefox windows or a window and an error dialog (Set as Default Browser does a lousy job creating ddeexec keys)

and the workaround answer is in this thread:
Firefox 0.9.x Incorrectly Setting Itself as Default Browser - MozillaZine Forums.

Let me know if the registry fix in that thread does it for you.

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