In that case I've recently discovered that the setting that disallows websites to choose their own fonts, browser.display.use_document_fonts = 0 , available in the advanced fonts dialog in the user-facing settings, seems to thread a middle ground of generally blocking web fonts, but still allowing icon fonts (perhaps due to icons fonts being Private Use Area codepoints that won't have local fonts?).
I've tried that, but unfortunately it breaks websites that use custom fonts for interface icons.