Good question! Actually (to my minor dismay): not completely. Disabling "font support" in Firefox surprisingly still has a hatch for "well-known" icon fonts, with intention to prevent "blind" icons in webpages. I believe it is driven by the pref

    browser.display.use_document_fonts.icon_font_allowlist

that contains "FontAwesome" and (Google) Material Icons and Symbols (many, presumably all, variants). So to truly disable all "non-preferred" fonts, we have to both wipe that pref and also change for the

    browser.display.use_document_fonts

to zero. But that's what the GUI checkbox controls, so no need to go to about:config for this one.