Maybe you could upload an image showing what you see the 'Addons' 'Extensions' list, so we can be sure it's correct. Then upload another image showing what you currently see in the drop down menu when you click on 'Tools'

Maybe my screenshot can help ?

Weird. Where you show the Message Header Toolbar Customize menu item, I just see empty vertical space that does not respond to clicks. I'll just remove the extension.

Well, after removing the extension, I still don't have the header menu, and the Tools menu still has the empty vertical space. I'm afraid of installing Thunderbird because I don't want to lose my folders and messages and configuration.

Maybe my report to the extension bug blog will be answered.

Installing Thunderbird shouldn't help.

Sometimes starting Thunderbird in troubleshoot mode helps AFTER removing the addon.

David, was the extension enabled when you looked for the menu item?

I did ask for images so we could check that A) the addon extension ' Message Header Toolbar Customize' was installed AND enabled B) we could see what was displaying in the 'Tools' drop down menu

David - we need you to co-operate and provide the info requested. We also need you to not change anything until we have seen and commented on the images.

So far, we know you keep installing and uninstalling the addon, but at no point do we see evidense that the correct addon is installed nor and more importantly we do not know whether it was enabled or not.

Please install the addon and upload images that show the requested info.

Suddenly, for no apparent reason, the message header menu showed up again today. I hope it stays.

Added: I think it showed up because I had to restart the computer due to the infamous "Windows slows almost to a halt" bug. So restarting is a workaround for this extension bug.