Some time back, Firefox decided to make their title bar short. As an aesthetic decision perhaps it made sense, and it does save a few pixels of real estate.
Recently, though, Apple continued with their habit of randomly changing things for no good reason and not giving people a means to revert. I hear it's a great way to cover up not actually having anything new to announce! On El Capitan, the old "maximize" button is now a "Take up the whole screen when that's obviously stupid because computers can do more than one thing at a time and we already had no trouble making things full screen for movies and the like" button.
Of course, Apple will helpfully point out that you can maximize by double-clicking on the title bar. The same title bar that Firefox made annoyingly short.
This can be easily remedied. Go to about:config in the url bar (as pictured) and search for
browser.tabs.drawInTitlebar. Double click it to make the value
True, and enjoy your new, more clickable titlebar for easy maximization!