From within TB, click help>more troubleshooting information. Then scroll down to Profiles and select About:Profiles. From that screen, select 'create profile. Then, create a name and, after that, browse to and select your profile. Click finish and TB should start working properly. That problem is usually caused by an anti-virus program interfering with a software update. Please advise if this went okay.

when i start TB, all I get is the "Home " tab with the "Welcome" screen, with choices for setup, and the "Account Setup" tab. No place to find the "Help" menu. Any suggestions for this scenario?

Click home. On next screen, right-click near top of screen and a menu will appear. Select 'menu bar' and then the menu bar will appear. And 'help ' is on that menu. I should have mentioned that earlier. Sorry.

Avast/AVG users are getting an alert about their prefs.js file, which stores important custom settings for Mozilla software. If you use Avast/AVG, the file may have been quarantined in the past few hours.

It's not yet clear exactly what is triggering the detection, so it's a bit soon to sound an all clear to restore that file from quarantine (if that is an option). Hopefully we'll know more soon.

Thanks, I'm making some progress, but I still have an issue when creating the new profile. When I select the folder for my previous (working) profile, I'm selecting "\users\[my user name]", but TB is insisting on using "\users\temp\" which is not where my profile is. Shouldn't TB be pointing to my user name, and not to TEMP in general? I do have norton installed ... known problem here?

Just to update this part in case it's relevant:'

It's not yet clear exactly what is triggering the detection, so it's a bit soon to sound an all clear to restore that file from quarantine (if that is an option). Hopefully we'll know more soon.

It was a false positive and you can use the Avast/AVG quarantine exception feature to restore the prefs.js to the original folder, overwriting the one Firefox created automatically to replace it. Related thread: https://support.mozilla.org/questions/1371872

Please review the users\yourID\appdata\roaming\thunderbird folder for a file called profiles.ini that is a text file that tells where the profile used to be. And should be where TB looks to find the profile. you are able to view and edit it when TB is NOT running. Check the users\yourID\appdata\roaming\thunderbird/profiles folder to see if your profile is there. The content of the profiles.ini file may be helpful. Please post a screenshot of the contents.