Instructions

Hide tab bar when using TreeStyle Tabs in Firefox

Taken partially from here.

Install TreeStyle Tabs.

Type or paste about:config in the address bar and press Enter/Return to load it.

In the search box above the list, type or paste userprof and pause while the list is filtered. If you don't see anything on the list, ignore this step.

Switch toolkit.legacyUserProfileCustomizations.stylesheets preference from false to true.

Type or paste about:support in the address bar and press Enter/Return to load it.

Note the Profile Directory.

cd to Profile Directory.

Copy this into the terminal:

                        mkdir chrome;
                        cat << EOF > ./chrome/userChrome.css
                        #TabsToolbar {
                            visibility: collapse;
                        }

                        EOF
                    

In Firefox again:

Restart all Firefox sessions.

Right click anywhere on menu section of Firefox, and check 'Menu Bar'.