In gnome-defaults-applications-properties, when selecting firefox as the default browser, the three "open link..." radio buttons are disabled. firefox by default opens a new tab in any running window, even when it's in another desktop, but using the -browser option forces it to open a new window, so there's a way to support this. I'm unclear whether this needs to be filed against gnome-defaults-applications-properties or against firefox.
This was done to fix https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=208371 If you want to change the firefox settings, change them internally to firefox for now. See the firefox Tabs prefs for the way to do it.