Rated 5 out of 5 stars

Actually it's not even an extension, it's just a CustomButtons button with such a code:
document.getElementById("appmenu-button").parentNode.appendChild(document.getElementById("toolbar-menubar"));
e.g. it just moves menubar as a child element to the app-button.
But now I've disabled that button (so the code doesn't execute), and the extension is still not working for me.

This review is for a previous version of the add-on (1.0.0.0).  This user has other reviews of this add-on.

First, this is an extension. I don't know what you think a CustomButtons is, I never heard that term, but I'm sure I coded an extension. Disabled AddOns aren't active, so there is no sense to check if it works after you disabled it. You can look at https://developer.mozilla.org/de/addons if you want more information about extensions from the developers point of view, there is a lot great stuff there.This AddOn does not move the menubar, nor it appends it as a child to the app-menu. The code you posted is completely wrong, too, you can access the correct code by unzipping the extension (xpi is a zip file with special files inside), 'bootstrap.js' in there is the relevant file. This extension changes the menu, it does not append the old one to the new one, and that is a bit more complex than a simple appendChild. If the AddOn doesn't work for you, communicate with me, so I have a chance to find the issue and fix it, if it is a issue inside the extension. You wrote about another extension causing problems in your last rating, it might help if you post what extensions you had enabed and what the exact problem is (for example 'the menu get changed in parts only' or 'the menu doesn't get restored after disabling the AddOn').