Right-click on the toolbar and choose “Customize” from the popup menu. In the screen you’ll get next, look at the bottom left. There is a checkbox,labeled “Title Bar”. Make sure that it’s ticked.
You may have to restart Firefox for the changes to take effect.
P.S.: You may also want to check out Plasmafox, which was specifically developed for Manjaro. More details in the quoted post below.
