suddenly without explanation everything disappeared.

Can you explain that in more detail?

I had exported a backup of the file in the profile subdirectory

What file? What export? You'd simply backup the entire profile folder.

I put Thunderbird in the trash to uninstall it and then did a new download.

That's a complete waste of time. The program installation is separated from the profile.

changed the name of the default file that existed because of the new startup.

Then what happened to your original profile? Re-installing thunderbird would not trigger the creation of a new profile.

Then I named the backup I put in the profile subdirectory the same as the one that existed at startup

What is the name of the profile folder? What is the absolute path to the profile folder? Please post the contents of your profiles.ini file (one level above the profile folder).

I can't explain why it is happening. I finally was able to rename my old file (that I found in Finders, I'm using a Mac Pro) as the default file and when I started Thunderbird I had all the files (local files with all the subdirectories I had created to put emails in) that had disappeared before.

I had one new email so I clicked on the "in box" and suddenly all the local folders and subdirectories disappeared again like the last time. I don't know why this is happening and I can't get them back. I only have one default file in my profiles directory so it can't be going to another file. 

I have always been unhappy that Thunderbird does not have a "save as" function (as do most other programs) so a person can save to some other media, i.e., flash drive, external hard drive, etc. and instead one has to go to the Finder and drag or copy the profiles file to another location to back everything up.

That is where I found the saved profile file with the name "ecbm7qu0.default" and replaced the profile file with it.

The name of the profile folder is "profile", with the above file in it. The absolute path to the profiles folder is "~/Library/Thunderbird/Profiles/~

Here is a copy of the profiles. ini file -

General] StartWithLastProfile=(0) [Profile0] Name=default IsRelative=1 Path=Profiles/ecbm7qu0.default

I don't know what happened to the text of the last reply but I was saying when I clicked on the new email all the local folders and subdirectories disappeared again.

I have always been unhappy that Thunderbird does not have a "save as" function

I was looking for a "save as" for all the emails in their respective subdirectories, not one that would have to copy each email separately. Is there a simple way to save all the folders and files other than copying the profiles folder?

For bulk saving, I'd try this add-on:

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

I have never understood the motivation for storing emails outside the mail client. Threading vanishes and following an extended conversation becomes near impossible.

The reason you want to save all emails in bulk is that if you have sorted them into subdirectories, like the name of certain businesses from whom you purchase supplies and there have been emails going to them for orders, then emails back showing the shipping and tracking information, you might want to refer back to them later if there is a dispute to show that there was a valid order, that the shipping took place, and the tracking order shows that it was delivered. The same for agreements that are not in writing any more but by email and are admissible in court as a "business record" because made contemporanous with the event so they are important. If you run a business you want to "document, document, document".

I still have the problem of getting the disappeared emails with their folders and subdirectories back. Thanks.

This problem is not solved. I still can't get to the multiple folders with subdirectories. Sometimes when I first start the program it will show them for a short time and then they disappear with only limited directories showing and not the ones I need. Where are they going to? The .ini file is apparently appropriate but the contents of the profile will not entirely load except momentarily. Is there a setting that is causing it? I have even downloaded the program again and still don't have access to all those previous folders I had backed up and then reinstalled in the profiles directory of /Library. Any suggestions?