Did Ubuntu made bet on a dead horse? I wonder...

[Roger]

<snip>
On Ubuntu 12.04 I've lost many mail. It just gets deleted. So, I also 
lose them on my other servers. I love the Linux idea but I still don't 
understand their modus operandi. I just can't depend on it long term
</snip>

I and my daughters have used Thunderbird for email and Firefox for 
working on line for many many years with no problem in both Ubuntu and 
Fedora.

Have you checked /.thunderbird/filename.default/Mail and all the folders 
therein.
The /filename.default will be something like c0abc24rt.default and there 
may be 2 or more of them.
I occasionally copy and rename the existing one and keep it as a backup 
for a week or so.

The /Mail folder will contain /Local Folders and your various mailboxes.
Each of these will contain a number of files and you can view Inbox, 
Drafts, Trash, etc

If these exist and you can view your emails in plain text then the 
problem is probably not Thunderbird, unless there was a fault in the 
installation.

In which case you can simply rename the .thunderbird folder to 
.mythunderbird or what ever then use the Add/Remove applications to 
delete Thunderbird.
  It will not remove the-renamed .file but will install a new one with a 
different filename.default.

 From here there are 2 options, copy your /c0abc24rt.default/Mail folder 
to the new one or alter the  .thunderbird/profiles.ini which contains:
[General]
StartWithLastProfile=1

[Profile0]
Name=default
IsRelative=1
Path=c0abc24rt.default
  or what ever it's called and simply rename that to the correct one in 
the .thunderbird folder the all your emails should be accessible.

either way, your mail if it exists, will be visible.

In any case practice making zip or Gzip backups of the .default file or 
even the whole .thunderbird file.
Rebuilding is easy, losing stuff is not.
Hope this helps
Roger