The window have a Firefox menu in the top-left corner and the menubar are hidden, but if you press the alt-key then the menubar appears.
Gnome devs are sometimes more interested in being zealots about philosophical concepts like "separating menus related to the application from menus related to the current window or document" that they don't eve see the effects their decissions have for discoverability, usability and productivity from an user point of view.
Here we have concepts that improve traditional menus and that do make real sense, like MegaMenus.
My issue with this appmenu idea is that it doesn't solve any real problems. Gnome has already been really consistent about the locations of actions into the menu. All this does is forces the user to think a bit more about which menu they're supposed to use, with the big problem being the menus could be separated widely in space.
I know they're been issues with multi monitor support in GS and this appmenu design only exacerbates it.
I'm telling you, if rhel desktop users report problems with these things they will be changed even if not by the gnome devs.
Wonder why Gnome is getting slower and slower these days compared to the other DE's?