If you're talking about the message header pane, you can't customize those buttons. The mail toolbar, on the other hand, is still customisable. You can right-click on the buttons themselves, such as the Write button, and it'll show the menu to customize the toolbar.

Thank you. I believe it is the header panel. For me "Delete" is too far to the right and "Forward" too far to the left. "Archive" is never used. I frequently press "Junk" instead of "Delete"! So a rearrangement would make life much easier.What a pity it can't be done.

Customisation of the header panel: This was reported as a bug as soon as it was forced onto users.

Recently, someone has taken up the case to try to improve that area, but I'm still not so sure it will have all the customise options again and the ability to add/remove or move buttons may need to be done later as a separate bug. Bug report below: https://bugzilla.mozilla.org/show_bug.cgi?id=1556261

However, there is an addon extension which may help with your situation, so give it go and see how you get on. https://addons.thunderbird.net/en-GB/thunderbird/addon/msghdr-toolbar-customize/

Thank you very much. That worked well.