Also using the snap on KDE and having a symlink downloads folder on another drive, I also could not send or save any messages, and the workaround (sudo snap connect thunderbird:removable-media) worked for me too.
Checking the Thunderbird error console previously showed "NS_ERROR_FAILURE: Component returned failure code: 0x80004005" when attempting to send or save a message, which is resolved with the workaround.
Also using the snap on KDE and having a symlink downloads folder on another drive, I also could not send or save any messages, and the workaround (sudo snap connect thunderbird: removable- media) worked for me too.
Checking the Thunderbird error console previously showed "NS_ERROR_FAILURE: Component returned failure code: 0x80004005" when attempting to send or save a message, which is resolved with the workaround.