According to the docstring:
font-menu-this-frame-only-p' is a variable declared in Lisp.
-- loaded from "/usr/local/lib/xemacs-21.0/lisp/auto-autoloads"
*If non-nil, then changing the default font from the font menu will only
affect one frame instead of all frames.
This variable gets set by the menu item
Options/Frame Appearance/Frame-Local font menu
But, at least for me, on XEmacs 21.0 "Poitou" it doesn't work -
using the font menu changes the fonts in ALL frames, regardless of the
setting of font-menu-this-frame-only-p
Y'all probably know about this already - I'm well aware that there are
lots of outstanding issues re: fonts and the options menu.
I'll try to work on a fix, but would appreciate guidance from somebody
who understands Customize a little better than I do.