Gmail IMAP contains a Trash folder and a IMAP/Trash folder. It seems the the IMAP/Trash is the current trash which Thunderbird deletes upon exit and the Trash folder is every email for the past 30 days. It this correct?
Well if you would like to delete something right away and you wanted to be gone immedietly you put it on the IMAP/TRASH if you don’t care when it dissapears then you set it on the other trash. For example if you just took a bunch of pictures in the nude and some you didn’t like or you a video having sex with somebody and you know that it was not the brightest of ideas then you put the trash in the IMAP/TRASH. I hope this makes sence.