Mozilla would like your help!
Here's the bug they are using to talk about adding KDE Global Menu support upstream: https://bugzilla.mozilla.org/show_bug.cgi?id=1419151
They are asking you to submit the patch to phabricator (https://phabricator.services.mozilla.com/) so they can include the patch in the regular Firefox releases (to support KDE global menu).
I'm currently trying to build Firefox with the unity-menubar.patch and I am having issues, currently I'm trying with the fix-wayland-build.patch which seems to modify the file which is causing problems. It's currently building but will take a while. I'll probably figure it out, but any hints would be welcome!
But yeah, the main point of this issue is not my building fiasco but instead to get your attention so you could get in touch with Mozilla. I appreciate your work!