The Header Pane buttons can be customized with an add-on, but they should appear by default. Help/Restart with add-ons disabled, check 'Reset toolbars and controls', 'Make changes and restart'. Do the buttons now appear?

Dear, We finally managed (my friend had corona, but he is better now) to checkout you suggestion. It does not work. He still does not get the header pane buttons after opening a mail. Is there another way to solve this problem? I get the feeling that by making the 'classic reader' the default setting it should solve the problem. The question is where/how can one configure this settings? Thanks for any help.

re :view via de classic reader

Does not sound like a default Thunderbird as the option you mention does not exist.

Is he using the addon extension called 'Conversations' ? The three vertical dot menu icon on the far right which has the 'view using the classic reader' option should also have the 'Reply' and 'Forwards' options.

It is possible the addon is clashing with another addon, but you would have to go through the routine of disabling all addons then enable Conversations, restart thengradually enable another addon - restart etc until the problem occurs.

I would consider why he is using the Conversation addon and whether it is necessary. You could uninstall the addon and use Thunderbird as it is designed. All those buttons would be there by default which would be same as the classic reader.

re :When opening an email my friend (85 years old) does no longer see the instruction bar which enables him to forward an email to his contacts.

....but those buttons may be below the message or offered in the right click options list. The problem seems to be that he is using an addon which deliberates offers a different view, but then does not prefer to use it. So you can see why I suggest removing that addon to see how he gets on.

Helpful info which is in addition to the info at the addon download webpage. http://write.flossmanuals.net/thunderbird/thunderbird-conversations/ The more menu at the top right corner of a message will allow you to perform operations on an individual message. The regular archive and delete operations are available. If you find out there's an operation that you can't perform with Thunderbird Conversations, just hit view using the classic reader. It will open the message in a new tab, using the classic interface, that might just have the option you're looking for.

At the bottom of the message, the reply / reply all / forward buttons allow you to reply to the message using the regular compose window. They do not use the quick reply feature discussed before.

Will re-verify if he has any add-ons running. I am puzzled that the option bar with reply, forward, archive, .... buttons (see my original attachment) after a software update does no longer appear on his screen. My installation of Thunderbird was done at the same time and for me it shows up. For him it does not show up, although he used it until the upgrade took place. He now works by activating the classic reader (via the rightside 3 dots) which brings up the wanted option bar. However this needs to be repeated each time he wants to act on a received message.

re :He now works by activating the classic reader (via the rightside 3 dots)

He must be using an addon extension called 'Conversations' as the three dots is not part of default Thunderbird, but is part of the addon 'Conversations' and activating the classic reader option is on a per email basis.

Uninstall 'Conversations' addon. There seems little point using an addon and then trying to stop using what it offers. The addon 'Conversations' classic reader option is basically what Thunderbird offers by default.

Thanks a lot for all help. The problem is now resolved. Thanks again contributors for the time spent. My old friend is again a happy man/user.