A few ligatures do have their own unicode code points, e.g. ffi, so the browser knows what is intended independent of the font. The ligatures used by icon fonts do not have their own code points.
Most icon fonts use private-use codepoints, not ligatures. These codepoints have no standard glyph, so disabling site-specific fonts will make those characters fail to render at all or display placeholder characters (e.g. �).
Thankfully, icon fonts are declining rapidly in usage - SVG icons are superior for most use cases.
> A few ligatures do have their own unicode code points, e.g. ffi, so the browser knows what is intended.
The precomposed ligature codepoints (e.g. U+FB00-FB06) only exist for compatibility with legacy encodings which included similar characters. They shouldn't be used in new documents. If you want text on a web page to use ligatures, use the CSS font-variant-ligatures property to control which ones are used. (And make sure to disable them on monospaced text!)
It does add a bit of performance gain not loading the fonts, and it may be a small security improvement, but if the goal was usability/readability that seems like a huge miss.
Add to that, this feature also needs to be understandable for non-technical people who might never have view a page source in their lives.
Much as a granular system would be more powerful and preferable for the HN community, having a dumb toggle makes a lot more sense from Firefox’s perspective.
This would be helpful for icon fonts, but also for user preferred fonts that don't cover all language character sets.
Apologies if I’ve completely misunderstood your comments
I wouldn’t be so sure about that. Resources like FontAwesome are still used heavily in VuePress and similar documentation generators.
edit: it may not be SVGs themselves. The icons on that site are repeated and I would reach for an SVG sprite that can be reused, I ran into issues with that working cross browser in XSLT - Firefox was actually the problem if I'm not mistaken.