I've been having issues with TB 91.11.0 (64-bit) on Windows 10 Home edition running on an Asus Celeron laptop. Many times I'm having issues with messages in folders not showing their subject information (the field is blank). Often the date field returns to the beginning of the Linux epoch. Using the Properties, Repair Folder usually restores the folders indexing, but often sets the status of many (often most) messages to unread.
Recently all the messages in all the TB folders suddenly blanked out. The folders still showed the proper count of the messages, and the correct count for unread messages, but nothing was displayed in the reading or header pane. Repair Folder would sometimes restore the information, but when I left the folder, the information would again vanish.
Eventually, I had to remove all the *.msf files and let TB reindex everything. This meant going through every folder and wait for TB to do the reindex as a foreground process. Even after doing all my ~70 or so folders, I still noticed some of them again had messages with showing blank subjects and bad message dates in the header section that had to be fixed with the Repair Folder option.
I'd like to suggest some administrative improvements to TB.
I've worked with many email clients, but TB is the first I've ever seen that has problems like this with message indexing. This is a major issue that needs to be addresses with a higher priority than pretty colors on buttons, or redesigned fancy menus. This is basic functionality that needs to be corrected as soon as possible.
A second time I've had the same problem with TB indexing failing and not being able to see the emails one or more folders. This is becoming an unusable product that focuses on trivial cosmetic improvements and ignores serious flaws in the basic processing. TB has to start focusing on the basic functionality and get this working reliably again.
I would like to thank BrianSW for this cogent description of this problem which I have been having as well. I would like to add that the main time I see errors is when I am using the portable TB version on a different computer. So far, the Properties -> Repair function has enabled me to recover almost all of the time. Now, I have to take extra steps when moving my portable TB to a different PC: First, use the Windows Properties Check tool on the media; next Compact Folders (I like that the Files Menu has a "Compact Folders" item that lets me select a main folder and all subfolders get compacted); next perform Properties -> Repair at least on subfolders that have been recently active. I do this when moving from PC 1 to PC 2, and then again when returning from PC 2 back to PC 1, and this has been reliable. I think you can see how having a "Repair Folders" item would be a big time saver.
Recently, I was seeing these same errors occur again. This time there was a more serious issue. I had created a subfolder, "Jobs", and a rule for moving job opportunities to the folder. I had also created a subfolder for a particular opportunity that had sent several emails with email links to forms to be filled out. Suddenly, this second sub-sub-folder vanished. Running properties -> Repair Folder did not recover everything. This sub-sub-folder was gone and not recovered. I began a frantic search and several other folders, including the Inbox suddenly were missing their contents as well. I ran the Repair Folder on each and eventually I found the sub-sub-folder in another location, No a subfolder of the Jobs folder, but a subfolder of another folder. I was able to move it back to the correct location and proceeded to repair other folders as well, including the Inbox several times. This seems to have resolved the issue for the moment.
What remains unresolved is that Thunderbird is unreliable when it comes to accurately managing email. This is a critical failure in basic functionality that must be addressed if this product is to continue to be used. Thunderbird must stop playing games with pretty icons/buttons/colors and get the basics of actually being able to process the user's email reliably.
I have experienced similar errors. The workarounds I mentioned have helped, but this is time consuming. Another workaround I can imagine, but have not tried, is just keeping the database very small, which kind of defeats the purpose for me. My smaller subfolders that have no recent activity have not had these errors. I agree that this unreliable behavior must be fixed and I am searching for a way to escalate this issue.