Rated 5 out of 5 stars

Lougout by oheil
Tested with Thunderbird Version 45.6.0

Worked perfectly. It's absolutely unbelievable that Mozilla never had this ability. Much thanks to oheil.

Right click on user account, click on Logout.
The next time you start Thunderbird, you'll be required to
enter your password, just as it did when you first installed it.

Problem solved at last

This review is for a previous version of the add-on (1.1). 

It seems that you are looking for something else: you can switch off the password manager in the options security tab. Than you will allways be asked again for a password when you restart thunderbird.

This extension is for the same result (thunderbrid asks again for password) WITHOUT restarting thunderbird!

But if you want to use the password manager in general but once in a while you want to be asked again for a password this add-on is what you need.

Anyways, thanks for the stars :-)