Currently, this can be done with CSS in author's web page or user's custom stylesheet etc
I'm not even sure a built-in mechanism exists for text fonts but maybe we want that for math fonts.
Note that font-family in Gecko is reset to serif and the actual selection of the default math font and fallback are listed in font.name.serif.x-math and font.name-list.serif.x-math configuration options. These options are accessible from the font preference menu of Firefox.
See also https://addons.mozilla.org/en-US/firefox/addon/mathml-font-settings/ for an example of add-on allowing font customization.
(In reply to comment #0)
> Currently, this can be done with CSS in author's web page or user's custom
> stylesheet etc
Exactly. We don’t need a new mechanism for this unless we have some idea about how to do it better.