If that doesn't exist, you could try this approach (the preference that the checkbox controls):
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.
(2) In the search box above the list, type or paste title and pause while the list is filtered
(3) If the browser.tabs.drawInTitlebar preference is currently false, double-click it to flip the value to true
For anyone landing on this page in 2020, please follow below steps on firefox:
ref: http://ubuntuhandbook.org/index.php/2018/05/hide-firefox-title-bar-ubuntu-18-0416-04
Make full screen (F11) then press ctrl-L or ctrl-K. Works at least for a while :)
If that doesn't exist, you could try this approach (the preference that the checkbox controls):
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.
(2) In the search box above the list, type or paste title and pause while the list is filtered
(3) If the browser.tabs.drawInTitlebar preference is currently false, double-click it to flip the value to true
https://support.mozilla.org/en-US/questions/1237317