Just reinstall from thunderbird.net. Just occasionally these issues appear here in the forum and reinstall has been 100% effective in the past.

What happens to the e-mails I have saved to local folders and to the various settings if I do a straight re-install? Won't they be overwritten? Where do I find the local folders in the Windows 10 file system? A backup of at least my locally saved e-mails might be prudent...

What happens to the e-mails I have saved to local folders and to the various settings if I do a straight re-install?

Nothing

Won't they be overwritten? Where do I find the local folders in the Windows 10 file system? A backup of at least my locally saved e-mails might be prudent...

See https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

Thanks. I'll see what happens!

An error message like XPCOM missing or about platform version or entry points not found can happen because of a failed Thunderbird update that left you with a mixture of old and updated files in the Thunderbird installation folder. This can happen in case security software was keeping a handle of some of the files in the Thunderbird program folder preventing Thunderbird from replacing all the files.

You need to reinstall Thunderbird and possibly remove the Thunderbird program folder to ensure that all files are replaced. You may have to disable security software temporarily in case the problem persists.

This can occur with desktop Firefox also.

It all worked fine. Thanks.