See https://support.mozilla.org/en-US/questions/1428027

That does not work. I've installed the Unified Folders Debugging addon, but no matter what the "Unified Folder" heading does not appear at all. I tried enabling various combinations of "All Folders", "Unread Folders", "Local Folders" etc, they all appear and disappear but nothing causes Unified Folders heading to appear. Even selecting everything causes all but Unified Folders to appear.

The three dots menus of the other headings only have "Move Up", "Move Down" and "Compact View", no "Reset Unified Folders"

See if you can make Unified Folders appear this way: Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, delete virtualFolders.dat, folderTree.json, and the Mail/smart mailboxes subfolder. Restart TB, select View/Folders/Unified Folders.

http://kb.mozillazine.org/Unified_Folders_not_working

After updating to version 115.3.3 (32-bit) a few days ago, Unified Folders don't show up for me at all. If I click on the three dots -> Folder Modes -> Unified Folders, nothing happens (it gets checked while the pop up is open, but if I open it again it's unchecked. Same if I do View -> Folders -> Unified Folders. I tried installing "Unified Folders Debugging" addon, and also tried opening Thunderbird in Debugging mode but nothing happens. There are no Unified Folders visible to try and right click and do things.

Thansk. This worked like a magic. Only observation : it seems that unified folder option may not work properly if the message data storage location for all mail accounts is not on same drive/location.