Hello! I've found that Firefox doesn't use startup-notification library. I cannot find any mentions of it in the code of firefox-75, besides one comment and .desktop file. revdep-rebuild -L libstartup-notification-1.so (and symlinks) doesn't mention firefox. It looks like GTK+ implements startup notification protocol itself, which could be seen in gdk_x11_display_broadcast_startup_message. Also, in Wayland environment this library won't work, for obvious reasons, and other means should be used. So, my proposal is to drop this requirement.
Oh, and in Fedora and Ubuntu firefox packages doesn't requires startup-notification
Oh, sorry, I haven't seen appropriate use flag!
It looks like in Firefox 74 upstream started using GTK+ API for startup notification (https://bugzilla.mozilla.org/show_bug.cgi?id=726479). And it looks like in Portage, latest Firefox has no startup-notification flag, so this bug could be closed.