have you also checked expunge on exit? Disk space in not release until you do that your manually select to compact folders which is becoming ever more difficult.

Hello Matt, thanks for you answer. Actually "expunge on exit" wasn't checked (I thought it had an effect only when you delete emails, instead I'm only trying to release space... deleting no emails) Anyway, I checked it, I compacted folders, get out of Thunderbird, get back in... and it worked for 2 of my 3 IMAP accounts. The 2 accounts for which it worked were gmail accounts; the third is not gmail... Have you any other idea..?

Thanks in advance

Hello again, maybe I've found a solution... When you right-click on a folder, you have a pop-up menu... choose "Properties": a box is opened with title "Folder properties". Below on the right, you find a button called "Repair Folder". When I clicked on it, a new download from the server for all the emails of the folder began, and the size of the corresponding file (e.g., "Inbox") decreased dramatically... It seems that the local copy of emails was deleted, and it was replaced by a much "lighter" copy downloaded again from the server. I think it's lighter because now only message headers are saved on my hard-disk...