Save your key pair (e.g., from Keychain or wherever) as a .p12 file, but do not save it with a password (or save it with a blank password). Try to import that .p12 file into Thunderbird via Preferences -> Advanced -> Certificates -> View Certificates -> Certificate Manager -> Import... It will fail with the error, "The PKCS #12 operation failed for unknown reasons."

I have the same problem, and it has to do with the existence of a password (or, rather no password). I have a StartCOM SMIME certificate in Mac OS X keychain. If I export it as P12 with a password, the import works. If I export it as a P12 with no password, the import fails with the "unknown reasons" error. Mac OS X 10.8.3, Keychain Access 7.0 (55121.4), Thunderbird 17.0.6.