Maybe the folder was accidentally moved somewhere else? Try expanding all the folder trees to see if it got moved.

Tried that.

Maybe you accidentally changed the folders view? Make sure View > Folders is set to All.

i had tried that too

If it's an IMAP account, you may not be subscribed to the folder. Control-click on the account name, and select Subscribe. In the subscribe window, make sure there is a check mark next to the folder you want to see.

POP account

Geez. LOL Okay, let's make sure the folder is there. Control-click on the account name, and select Settings. Within the settings for that account, click on the Server Settings panel. At the bottom-right corner, there's a field called "Local Directory". That's where the messages for that account are stored.

Using Finder, go to that folder. Your mail folders are contained in UNIX mbox format. They are the files without extensions (Inbox, not Inbox.msf). Is there a corresponding mbox file for the folder you're looking for?

If so, the file that stores the folder layout may be corrupt. If you delete it, Thunderbird will recreate it the next time you run Thunderbird.

1. Go to Help > Troubleshooting Information, and click on Show in Finder. That will open your profile folder in Finder.
2. Close Thunderbird. (It needs to be closed when these changes are made)
3. In your profile folder, rename the file panacea.dat to panacea.dat.backup. (Instead of deleting the file, this way you can restore the file if these instructions don't fix the issue.)
4. Run Thunderbird to see if the folder reappears.

went there. There is a folder called inbox.sbd which contains the folders. should I delete inbox.sbd or just its contents?

Don't delete either. Do the following...

1. Go to Help > Troubleshooting Information, and click on Show in Finder. That will open your profile folder in Finder.
2. Close Thunderbird. (It needs to be closed when these changes are made)
3. In your profile folder, rename the file panacea.dat to panacea.dat.backup. (Instead of deleting the file, this way you can restore the file if these instructions don't fix the issue.)
4. Run Thunderbird to see if the folder reappears.

did what you said. closed thunderbird renamed panacea.dat to panacea.dat.backup opened thunderbird and the folders are still NOT there i re-ran Search Messages and i can still see the emails and folders but they are not in the left sidebar

Try this:

1. In Thunderbird, create a sub-folder called TEMP
2. In Finder, go to inbox.sbd folder, and close Thunderbird.
3. Delete the files TEMP and TEMP.msf.
4. Rename the files for your missing folder to TEMP and TEMP.msf.

You are not going to believe this. I created Temp and Temp and my missing folders appeared.

How did this fix it? Do not know but, unless they disappear again, I am satisfied. Thanks for your patience and suggestions.

My folder shows up in a Search in the Filters under Folders by its folder name followed by "(Local Folders)". I'd gladly transfer it into a new folder, but don't know how. I left a message under How to Create a Saved Search Folder because it doesn't show how to do the Match by the Filter -- or if it does, I'm not understanding it.

You're replying to a thread that is old. You should use https://support.mozilla.org/en-US/questions/new where volunteers can get more details about your setup.

Because this thread is so old and more people might start using it for their own questions, I'm going to lock it.