Try this: Help/Troubleshooting Information, click Open Folder to open the profile folder, close TB, delete session.json, restart TB, and see if the Folder Pane setting is retained between restarts.

Thank you for your suggestion. I deleted the session.json file as recommended and the behavior persists. I also tried creating a new, empty .json file, made sure my folder pane was open, closed TB, re-opened and the behavior still persists. I opened the .json file to review and see this text listed there: "folderPaneVisible":true...implying that the display state is defined properly, but not being honored. Hmmm...

The 'xulstore.json' file has the information on what size the FolderPane width should be. eg: "folderPaneBox":{"width":"290"},

Possibly the 'xulstore.json' file is incorrect and not getting updated or perhaps is corrupted and not able to update when Thunderbird closes.

Do you use CCleaner or similar product that cleans up files ? If yes, please make sure CCleaner does not have access to any Thunderbird folder or file.

Otherwise:

• Help >Troubleshooting Information
• click 'Open Folder' to open the profile folder
• Exit Thunderbird now
• delete 'xulstore.json' file
• restart Thunderbird.

Set width of FolderPane Exit thunderbird.

Access the profile folder, open 'xulstore.json' file using a text editor like Notepad. Look for: "folderPaneBox":{"width":"290"}, Does it now have a proper width ?

I don't think the xulstore file is corrupted, as you found the problem is there in safe mode, which applies a default layout, i.e. default xulstore.

Delete foldertree.json and directorytree.json from the profile folder (they are automatically recreated) and see if it makes any difference. If these simple steps don't help, in these cases it's often faster and less aggravating to create a new profile with Profile Manager: Winkey+R, type thunderbird.exe -p

@Toad-Hall - Thank you for your comment. I tried your suggestion regarding the xulstore.json file and whether the folder pane is visible or not (I tried both conditions), xulstore.json includes a proper declaration for "folderPaneBox". When I tried closing TB with the folder pane visible, the width was defined as 323. I then restarted TB with the folder pane closed and the width was defined as 125...even though the pane was not visible upon closing TB. I did notice, however, that this (seeming) default value was the width of the pane while it was briefly visible upon relaunch...much narrower than how I typically adjust it manually. I do have CCleaner installed, but it does not launch with Windows, does not have automated functionality enabled, I only use it for registry cleaning and even then I'm very careful about changes I allow it to make. I use it maybe 2-3 times a year, and not since well before this issue with TB came about.

@sfhowes - I tried deleting the two .json files you recommended, but the behavior persists. I'm likely going to try to create a new profile, but need to make sure I do it in a way that saves my content - and mail account settings if possible as there are 4 currently in use. Two of these include emails back to 2007 that I would like to retain. I've always just backed up the entire profile folder and copied it to a new system/install as needed. But since I'm changing profiles on the same system, I'm not 100% on how to retain only what I need while getting rid of files that are likely corrupted. I'll be looking into that in the coming days. Thank again for your input, much appreciated.