Here are screenshots showing the problem that I spoke of above. You can see that I cannot expand and view the folders of 3 email accounts when in the unified view.

On my system, with Unified view, the only accounts that are 'expandable' are the ones that have custom folders defined, as opposed to the standard folders such as Sent, Trash, Templates, Drafts etc. In your first picture, the 'hope' account only has the standard folders, which are displayed in Unified view under the unified Inbox, Drafts, Sent and Trash folders. I suspect the 'expandable' accounts only show custom folders when expanded.

I performed this test after deleting the smart mailboxes folder in the Mail folder of the profile, and also deleting virtualFolders.dat and folderTree.json. This is the usual remedy for malfunctioning Unified folders:

http://kb.mozillazine.org/Unified_Folders_not_working

I'm not exactly sure how to do what you're asking me to do. Is this something that I can do within Thunderbird? Where do I find the Mail folder of the profile, virtualFolders.dat, and folderTree.json? After deleting these folders do I then have to recreate them in order to receive email or will Thunderbird or my server automatically recreate them for me?

I found my profile folder and deleted the mail folder, virtualFolders.dat, and folderTree.json. I then closed Thunderbird and reopened it but nothing had changed. The folders still are not visible in the Unified view.

Yes, something did change. Now all of my emails are gone! At least nothing shows up in my unified inbox. I would rather have it the way it was. It's even worse now.

You said you deleted the Mail folder, when the instructions stated to delete the smart mailboxes subfolder of the Mail folder. If that's what you did, then you did in fact delete the messages. Perhaps you can restore them from the Recycle Bin.

The suggestion to delete smart mailboxes and the other files was simply to rule out any existing issues with Unified folders, i.e. start with a clean version. My result showed that accounts are only expandable in Unified view if they contain folders that are not displayed in the unified Sent, Trash etc. folders. In other words, you can have a Unified folders view or All folders view, but not at the same time.

I restored all three files/folders (smart mailboxes, VirtualFolders.dat, and FolderTree.json). Now everything is back to the way it was previously. (I had not deleted the mail folder, but the smart mailboxes as you instructed.) I still cannot expand the three accounts in the unified view. All three of these email accounts are hosted by Hurricane Electric. I am not having any trouble with my Gmail accounts or my Yahoo accounts. Could the problem have something to do with the way Hurricane Electric does their IMAP mail?

https://faq.he.net/index.php/Email

I don't think it's related to how HE configures its IMAP server. Try this experiment: create a folder under one of the accounts that is not 'expandable' in Unified view (right-click the account in the folder pane with All folder view, New Folder, name it). Now, in Unified view you should see the account is now expandable, and when expanded it shows the new folder.

That is what I explained above: accounts are only expandable in Unified view if they have user-created folders, and then they only display those folders; the standard folders are already shown in the unified Inbox, Sent, Drafts etc.

I created a folder and, just as you said, the account was expandable and that folder was visible in the unified view. I checked the inbox of the unified view and confirmed that the emails in the standard folder inbox for that account were showing up. I guess this means that everything is working the way it is supposed to work? If that is the case, my problem is solved. (Or demonstrates that I never had a problem at all.)

I think that's just the way it's meant to work: all the folders are available in Unified view, but there's no unnecessary duplication. It's not immediately obvious that should be the case, and I'm not aware of any documentation that explains it in detail (this thread may be the first).

Thank you for all of your help with this. It was great.

After thinking that my problem was solved, I continued to have problems. Yesterday many of my folders simply would not show up. I tried unsubscribing and subscribing again. I tried refreshing. I did everything I could think of but they wouldn't come back. Finally I uninstalled Thunderbird and re-installed it. The first account that I entered showed all of my folders (even the standard folders). The other accounts that I entered would not show all of my folders no matter what I did. I couldn't even get it to show the folders that were shown in my first installation! I unsubscribed and subscribed again, but it did no good. I refreshed, but even that would not show my folders. I deleted the accounts and recreated them again several times, but it did no good. I have concluded that the problem must be either with the Thunderbird program or with one of the add ons that I have. I have installed gcontactsync, Lightning, Mail Merge, Manually sort folder, and Provider for Google Calendar.

I am going to uninstall Thunderbird again and start over. This time I will not install any add ons and see what happens.

Please help me! If the problem is with one of the Thunderbird add ons, other users should be aware and those add ons should be removed until the problem is corrected. If the problem is with Thunderbird it needs to be brought to the attention of the developers.

Uninstalling and reinstalling the program itself seldom helps, as all the user data is stored in a separate profile folder, and the profile is where most problems occur. Better to create a new profile with Profile Manager, set up the accounts, and then transfer mail, contacts etc. from the old profile, and install add-ons, incrementally, to avoid copying incorrect data from the old profile. You can run TB in either profile by selecting one when PM starts.

To start Profile Manager, close TB, Winkey+R and type thunderbird.exe -p

You could also run your original profile in safe mode (hold Shift when you launch TB), which disables all add-ons, among other things, and see if the accounts work properly.