Because of some other bugs with Mail.App's handling of certain email formats, I've installed Thunderbird to give that a try. I configured it to be the default (from the Mail.App preferences). That seems to work.
BUT if I search for an email via Spotlight (Thunderbird's search is...well, I'm using Spotlight), clicking on a found mail item from Spotlight still launches Mail.App.
I suspect it's a setting in a plist; how can I convince Spotlight to open Thunderbird instead of Mail.App?