As you can probably see in these screenshots, the menu is not integrating into Unity.
I guess that's what I get for using a deprecated DE, but I'm holding out hope that there's a fix.
Officially, non of Mozilla products support the global menu (https://bugzilla.mozilla.org/show_bug.cgi?id=1419151). The support of the global menu is in first place something that need to be supported by the toolkit and like Firefox have his own toolkit it need to be support that internally. So, how the support for the global menu in Firefox is not included by default, this is included then thanks to some externals developers. To be more specific, this was supported on Ubunt-Unity only because the Ubuntu developers merged a patch with the original source of Firefox. Also the same patch was apply in several linux distributions after that. You can find the original patch here: http://bazaar.launchpad.net/~mozillateam/firefox/firefox-trunk.head/view/head:/debian/patches/unity-menubar.patch
Now that the Ubuntu developers drop Unity in favour of gnome shell, we can assumed that they will drop also the support for all this type of patches. So, the new versions of Firefox in Ubuntu will probably have soon a not updated patch that really will NOT work with the new sources of Firefox and will be retired.
This is not just for Firefox, but it's for a lot of applications that already have not a global menu in an officially way. In this applications the global menu is supported only because some external developer to that applications created a patch for it and probably they was also some Ubuntu developers. See as an example: https://bugs.launchpad.net/ayatana-design/+bug/1131664 Also please note in particular the comment #61 of the previuslly link that confirms the direction of the Ubuntu developers about that.
In your case is probably worse because you are trying to run Firefox directly from an upstream source, but Firefox like a lot of officially GNOME applications have not upstream support for the global menu at all.