Type about:config in the address bar and press Enter to load it.
In the search box above the list, type userprof and pause while the list is filtered. If you do not see anything on the list, ignore this step.
Switch toolkit.legacyUserProfileCustomizations.stylesheets preference from false to true.
Type about:support in the address bar and press Enter to load it.
Note the Profile Directory.
cd to Profile Directory and create a folder called chrome.
***
mkdir chrome;
cat << EOF > ./chrome/userChrome.css #TabsToolbar { visibility: collapse; }
EOF
***
In Firefox again:
Restart all Firefox sessions.
Right click anywhere menu section of Firefox, and check 'Menu Bar'.
