User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:52.0) Gecko/20100101 Firefox/52.0 Build ID: 20180621064021 Steps to reproduce: I upgraded from Thunderbird 52.8.0 to 52.9.0. Actual results: On one of my clients (Windows 10) I get bogus offers for compating the IMAP mail folder: The folder is about 170MB in size, but the first time thunderbird claimed that about 250GB of space could be freed. I then accepted the offer, which resulted in all mails from my inbox being downloaded again. Ever since then, I regularly get new offers to compact my mail folder. This time, the size that could be freed is smaller (it started at 1GB or so), but with each offer the amount increases (now it claims that 5GB can be freed). This does not happen on a Windows 7 system using the same IMAP mailbox. Expected results: Compacting the folder should not be offered every few hours with bogus folder sizes.

sounds like bug 1473302

You an switch off that offer: Tools > Options > Advanced, Network & Disk Space: Compact all folders when it will save over ...

BTW, does the problem go away if you repair the folder? Right-click on the folder, Properties, Repair Folder.

I can try on Monday, but given that this is a very fresh install, I wonder how broken it could be after just two months, while the other client without the problem has been using its folder for years...