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?
Best Answer
The best way I've found to set any default app is with RCDefaultApp - freeware - old but still works in Yosemite.
It installs as a Control Panel item called Default Apps; you can then set many defaults - simplest are the 'Internet' ones, first tab - dial in your preferred app from the drop menu.
e.g.