[Originally incorrectly posted to Firefox forum] Thunderbird uses mbox (single file per folder) storage by default. This is not a good arrangement when some folders grow to 2GB over the year; each backup requires the entire file, and that becomes 1GB and more per day for each of those folders for much of the year, ending at 2GB per day each. I find some mention and discussion of maildir (one file per message) with Thunderbird (as a plug-in). But most of what I see is ancient history, and a lot of it warned of significant problems and dangers. It was 'experimental' for Thunderbird 60, and nothing said subsequently. I see some success claimed from 2016 (Wilders Security). That's old. The Mozilla Wiki warns of dangers and data loss, last updated 2017. Support Mozilla (undated) says you risk your data. http://mzl.la/1GDeRsp Is there any up-to-date information? Does it work, reliably? Can I bet my data on it? Or is maildir in Thunderbird a no-show?