/usr/lib/firefox-addons/extensions/ is in firefox-globalmenu 11.0+build1-0ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome.manifest
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/globalmenu/
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/globalmenu/content/
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/globalmenu/content/firefoxMenu.js
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/globalmenu/content/firefoxOverlay.xul
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/globalmenu/content/thunderbirdMenu.js
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/chrome/globalmenu/content/thunderbirdOverlay.xul
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/components/
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/components/components.manifest
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/components/globalmenu.xpt
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/components/interfaces.manifest
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/components/libglobalmenu.so
/usr/lib/firefox-addons/extensions/globalmenu@ubuntu.com/install.rdf