The stock firefox has never supported a global menu — or at least, not on GNU/Linux, because the macOS version does — but there are specially patched versions of it in the AUR, and likewise for some of the firefox spinoffs like librewolf.
I actually have a HowTo about it in the Tutorials section. 
The problem however is that it takes about 5 hours of compiling. Been there, done that, have the T-shirt and the bumper sticker. 
Well, it’s a matter of personal preference. The global menu corresponds to my personal preferences in ergonomics and logic, and it makes for a more integrated feel regarding applications and operating system, which is perfectly consistent with the fact that UNIX was conceived as an integrating toolbox, in which applications become mere extensions of the operating system, as opposed to separate entities like in MS-Windows.