Mozilla Maintenance Service allows Thunderbird updates to occur in the background, without requiring you to click Yes in the Windows User Account Control (UAC) dialog.

Is it needed? If you don't want to be nagged by Windows, yes. :) If that doesn't matter to you, you can disable it.

More information: What is the Mozilla Maintenance Service?

Did I understand it correct? If you want to make use of it: so you don't want to click yes in UAC and you select "make use of backgroundservice to update" in the options Then you need to update the Maintenance Service to the version equal the TB version?

Almost correct. Mozilla Maintenance Service get updated with Thunderbird. Most people aren't even aware that it exists.

If you have any more questions, let us know.

thanks for your answer.

this means there went someting wrong during my semi-automatic update from 68.1.2 to 68.2.0

I said semi-automatic because I opened help->about and clicked "renew".

If, for any reason, you feel that the update did not work correctly, you can always download and run the installer from https://getthunderbird.com . When you run the installer, Thunderbird will pick up your existing data.