AND, your instructions; "Recent versions of Thunderbird and Firefox have a about:profiles link in Help -> Troubleshooting Information. Clicking on that link launches an enhanced version of the profile manager. That is the easiest way to launch the profile manager. If you do that you don't need to read the rest of this KB article."; do not work. It does say "Troubleshooting Information", it says "More Troubleshooting Information." And it opens an information page about the current profile, but nothing to do with the "Profile Manager." I can see my old profile in Windows Explorer, but there is nothing that easily gets me to the Profile Manager to choose the old profile.

Answering my own questions here. On the Profile Information page I found a link called "About Profiles" down the page. I click on that and it showed me the two profiles with an option to create the old profile as the default. If you don't hear anything more from me, then I have solved the issue. Firefox needs to rewrite their instructions. It is easy if you are told the proper place to look.

This did not solve the problem.

Hi, have you looked in your profile directory? The old profiles should be there... depending on your operating system there should be a folder where they are. about:profiles should tell you more.