Yes, that was it! I had History set to "never remember history," which did not allow me to use custom settings for history. I instead checked "use custom settings" and sure enough, it had "always private" checked, which I unchecked. Now I can enable Notifications here. So it seems that in order to use groups.io notifications in FF, you have to enable history, at least in the current incarnation?
Unfortunately, though, doing this made it so that when I double-click on FF in the dock, it no longer allows me to open a new window from there.
Messages are the sole opinion of the author, especially the fishy ones.