Firefox tabs in title bar under Linux
Firefox on Linux displays the tab bar below the title bar by default. In order to move the tabs to the title bar, like Chrome and Firefox on Windows do, open about:config and set the configuration key browser.tabs.drawInTitlebar to true.
browser.tabs.drawInTitlebar = true
The effect can be seen in this animation:
Note: This key was formerly called widget.allow-client-side-decoration and has been renamed in a recent version of Firefox.