Almost all the tutorials describing how to do this describe obsolete menu arrangements that do not correspond to the Thunderbird I run
Click on "hamburger" icon to get a menu. Click on "preferences" to get that menu. Select advanced on the top right Select Network and Disk Space tab Uncheck the "compact when it will save xxx"The Thunderbird dorks will scold you for doing this. They say that "compacting" is part of regular Thunderbird Hygiene. I say that if so, it should be automated. Or there should at least be an option to automate it.
What compacting does, as far as they have managed to explain it, is to delete already deleted mails (i.e. "empty the trash"). I does not compress anything, it just gets rid of dead wood. They warn you that Thunderbird is buggy and may loose mail if this is not done on a semi-regular basis.
Adventures in Computing / tom@mmto.org