You cannot mix S/MIME and OpenPGP in the same message. This will fail as the two standards (and the implementation in Thunderbird) interfere with each other. If you want to use S/MIME you should not enable the Enigmail option "encrypt if possible" (nor the one from S/MIME).
In order to support our users, and especially for fixing defects, we often require a debug log file. Enigmail contains a function that creates such a log file if needed.
Open the Enigmail Preferences panel (menu Enigmail -> Preferences)
Activate the checkbox Display Expert Options and close the dialog
Open the menu Enigmail > Debugging Options > View Log.
Click the Save Log to File ... button to save the log.
Since version 1.8, Enigmail can decrypt mails permanently.
The handbook describes how to do this by using mail filter rules. In addition, version 1.9 added a context-menu option which can be used on the fly. Use the right mouse button on a message in the message list and select the option Decrypt to folder.