The only method is to individually remove unwanted email addresses.

However, if you are removing most then it might be just as easy to use 'Reply' and add the email addresses you want using the 'contact sidebar'. You can hold down 'Ctrl', select contacts and then click on the 'Addto TO' or other option. This adds all selected addresses in one go.

But 'Reply-All' assumes you actually want to 'Reply to all'. So, I suppose the best method is to ascertain whether you are removing a few or most and tailor the use of the 'Reply' or 'Reply-all' to suit.

Not a good solution since not all recipients are in my contacts and I should put individually the missing ones and then choose the good ones from a very long list. I have a better, but weird solution: Look at the source of the message, copy the list of recipients to any text editor, edit it easily, then copy the list to the reply. It works, I've tried it, but I hoped in a more user friendly solution. However thanks for the advice.

Ultimately, only you can know which to keep and which to discard.

I use an add-on that puts a X into each address box, so it's a simple one click to remove any address. It doesn't solve your problem but might make deleting addresses less fiddly.

https://addons.mozilla.org/en-GB/thunderbird/addon/address-close-button/

Installed! It works fine and is (almost) what I needed. Thank you very much !!