User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36 Steps to reproduce: I do not know if this is easily reproducible. I have Thunderbird set to compact folders when it will save more than 20 MB of space. Thunderbird is set to automatically update. Current version is 52.9.1, 32-bit. All accounts are IMAP. I have 5 Gmail accounts, one Yahoo account, one AT&T account, and 6 accounts that are hosted with 1and1. I have local folders for archive of each of these accounts, set to yearly archive folders, keeping existing folder structure. One of my Gmail accounts (the main one) had over 50k messages in the Inbox and many more in All Mail. I filtered messages from Inbox to various subfolders (labels in Gmail), then archived everything before 1 Jan 2017. I also archived messages before that date in All Mail, afterward. I filtered to subfolders, then subsequently archived mail before 1 Jan 2017 in each of the other accounts, as well, but there were significantly fewer messages in each of these, except the Yahoo account. At some point, I started to individually compact folders immediately after archiving their messages. When this was completed, I backed up my Profile folder, then removed the archive folders, unsuffixed files, and msf files that were not 2017 or 2018 in <profile_name>/Mail/Local Folders/<accountname>.sbd. The default profile folder size was 18 or so GB before that, and it is now 10.7 GB. Since this time, I have received the auto-compact message with varying "This will save about..." sizes, but those numbers seem to be growing. I have not added any more mail to the archives, and no more mail than usual has come in via the various IMAP inboxes. Actual results: Periodically get pop-up prompt to allow auto-compacting with following text: "Do you wish to compact all local and offline folders to save disk space? This will save about 242 GB." Pop-up prompt is desired behavior. Bug involves the text. The Profiles folder is only 10.7 GB. Click to allow compacting and same pop-up appears later, with a similarly-sized "This will save about..." which keeps increasing in size. For example, when I started this report, I'd just seen a pop-up saying it would save 242 GB, but just now (a bit over 24 hours later), it's saying it would save about 255 GB. This does not seem to be expected behavior. Expected results: Periodically get pop-up prompt to allow auto-compacting with following text: "Do you wish to compact all local and offline folders to save disk space? This will save about <something less than Profiles folder size>."

I have a couple other screen shots to share, if that would be helpful. One is of the profiles folder properties window, showing the size of that folder, and one is of the folder pane of Thunderbird, showing the sizes of each of the accounts, except for all but one of the Gmail accounts. Two of the Gmail accounts actually have next to nothing in them, as they're not active, and I'm just keeping them in Thunderbird to monitor them, in case something comes in to them. The rest SHOULD have some size, but two of those don't show anything but a dash in the "size" column, and from the "total" column, it's obvious that "size" should not be empty.

Thanks. This is a known, but unresolved issue - bug 1260698