This is Fedora 30 with Xfce 4.13 The fonts settings currently makes no difference for Firefox (and probably Thunderbird) as the latter only obeys fontconfig settings. Firefox here was terribly ugly with the default system settings (just hintslight and nothing else). This was good in the sense that I finally learned how to configure fonts (it was just a matter of linking the desired conf files from the available ones - in Fedora from /usr/share/fontconfig/conf.avail/ - to etc/fonts/conf.d/). But it was not only ugly, it was very uncomfortable to use and ideally this definitely should be dealt by the DE settings. I suspect GNOME does that. So, unless I'm mistaken, the appearance font settings probably should create a $XDG_CONFIG_HOME/fontconfig/conf.d/local.conf file or, in case the conf.avail directory is always in the same place regardless of the OS, link the selected conf files to $XDG_CONFIG_HOME/fontconfig/conf.d If this is ever implemented, then maybe also add the lcd filters option.
Although, thinking better, as annoying as it is if it's only Mozilla's apps that don't use the font settings (actually I only checked Firefox) then the bug is in there, not in Xfce.
I opened a bug report for Firefox at https://bugzilla.mozilla.org/show_bug.cgi?id=1563399