Manual updates are now enabled, but for the en-US locale only.

Does Thunderbird auto-update all its clients simultaneously?

No.

Should I wait or download the latest version manually?

That's up to you. Unless there's an urgent need, I'd wait until you receive the manual update.

I have a copy of the Thunderbird profile from my desktop computer that is now running version 91.1.2. Can I restore that profile to my laptop, which is running version 78.14.0?

Can I restore that profile to my laptop, which is running version 78.14.0?

I'd rather upgrade to TB91 on the laptop computer too, prior to restoring the profile. Profile downgrades are likely to cause problems, and are not straight forward.

If your locale is en-us the build-in update mechanism should work by now via menu button - Help - About Thunderbird. Other locales will follow.

It doesn't hurt to take a look at the release notes. https://www.thunderbird.net/thunderbird/releases/

Thanks. Yes, normally I would have preferred to upgrade my laptop to TB91; but I found that TB91 gave problems on my desktop, which have not yet been resolved even with upgrade to version 91.1.2. I have posted these issues but have had no response. Version TB91 provoked loss in Windows 10 of the shortcut facility of "Send to mail recipient"; the link seems to have disappeared. Also if I wait too long to send or save an email after making an attachment, TB91 tells me that the file can't be attached because I don't have the necessary authority. Best Philip.