Try the 'Extended reply header' option of this add-on:

https://nic-nac-project.org/~kaosmos/changequote-en.html

http://chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html

I downloaded and installed the add-on as indicated. I hate the result.

1) the add-on does not appear in the list of add-ons. So I can't uninstall it

2) the reply header is worse than before. The time of the original message is no longer mentioned. It used to be in English, it is now in my browser's interface language (Dutch).

3) I had a multiligual localisation switch add-on. It has now disapeared.

On the basis of 1) and 3) I suspect your add-on of doing something weird during the installation On the basis of 2) I infer that your add-on is doing something, but definitely not what I expected.

How can i repair this mess?

In Thunderbird, Tools|Add-ons|Extensions - it really isn't there? You didn't look in Firefox by mistake? ;-)

I'm a bit wary of this add-on. I've only ever used it in an all-english environment, so I can't comment on any adverse effects on alternative locale settings. I found it rather prone to quoting the wrong message body, and it didn't always honour my header settings.  :-(

I suspect you can set the "standard header" via the Config Editor. I can't recall mine ever using the format you describe, so that is empirical evidence that it can be changed, though maybe my aborted test of this add-on changed it for me.

Look for mailnews.reply_header_authorwrote and related config entries.

It's even worse than I thought. This extension has taken a total grip on my preferences. I can't change anything. Everything reverts to what the extension thinks it should be. If I use the configuration editor, I can change what I like, it is never saved. My Prefs.js keeps a constant time stamp.

The only solution I see is to uninstall Thunderbird, delete its folders, and reinstall it.

The add-on works fine here, and shows none of the ill effects you describe, so I can only suggest your preferences file is corrupted and can't be edited. You could create a new profile with Profile Manager, set up your accounts, and transfer the mail and address books from the old profile.

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird

OK, I'll create a new profile. I will have to transfer the contents selectively and by hand, because if I copy the lot, I will also transfer the bug! I intend to transfer everything, excepting the add-ons. I will then reinstall them.

This looks like a lot of work. I don't have time right now. I might do it over the week end.

Please keep me posted if you get some new ideas.

Thanks for the help. I hope it will help.

could you please post the Trouble shooting information from the help menu (alt+H) while using your old profile.