I somehow lost the "global menu bar integration" firefox addon. Probably uninstalled it accidentally.
I have tried
dpkg-reconfigure and also
apt-get remove firefox-globalmenu && apt-get install firefox-globalmenu. But neither worked.
How do I get the add-on into Firefox addon?
I couldn't find an xpi file either, to point firefox manually to it for installation.
Edit: Found a fix on my own, see my comment to the answer that was mots helpful :) Thanks everyone for looking into this.