Yahoo has updated its Desktop Search and now supports Thunderbird email. Although previous versions of YDS did index and search Thunderbird files you had to search them under the “All files” category. Now you can chose just “Email”. The number of file types supported in the main program has been considerably reduced, though. If you want the full range of 300+ file types you have to install the expansion pack.
For me, Yahoo Desktop is still by far the best with Copernic’s offering coming second.