User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:220.127.116.11) Gecko/20071127 Firefox/18.104.22.168 Build Identifier: 22.214.171.124 (20071031) german version I use 2 IMAP accounts with ~30 folders combined and have some filters to sort the messages. When I select "compress all folders" (German: "Alle Ordner dieses Kontos komprimieren") from the file menu, this will result in heavy fragmentation of the mailbox files. I just checked a ~150MB mail folder: one disk fragment before compression, ~1200 fragments afterwards. No mailbox ended up in less than some dozen to several hundred fragments. On a related note (different issue?), the compression does not seem to finish. Disk activity stops after a while, but there is still a message in the bottom left corner: would be something like "checking for new mail" in English. I have not checked this time, but several times I noticed there were temporary files in the mail folders, although judging from network & disk activity, the checking for new mail & folder compression had finished. I am not an expert, so please be patient if I should have overlooked something obvious. Reproducible: Always Steps to Reproduce: 1.Check fragmentation of mailbox files (I used "Defraggler" which shows fragmentation for individual files). Defragment them if applicable. 2.Start Thunderbird & execute "compress all folders" from the file menu. 3.Check fragmentation again. Actual Results: The mailbox files consisted of anywhere from ~10 to ~1200 fragments. Expected Results: The mailbox files should not be fragmented that heavily (if at all).