The bookmarks menu is still somewhat wonky because it's all async but at least I get mostly working submenus in Firefox now. Both title bar and appmenu applet. Didn't notice any issues with other already working apps. If someone could test whether Electron apps still behave well with it.
{F5615267}
https://bazaar.launchpad.net/~mozillateam/firefox/firefox-trunk.head/view/head:/debian/patches/unity-menubar.patch#L955