bugzilla.mozilla.org/show_bug.cgi?i
Firefox developers shouldn't expect sites to support it if they always take the approach of trying to blame limitations of the web browser on web developers...
I spent substantial time researching this and coming up with a way for them to fix the issue.
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I think html.spec.whatwg.org and html.spec.whatwg.org/print.pdf really drive home the point. The former uses an SVG favicon and the latter falls back to a hard-wired request for /favicon.ico resulting in a 404 and a placeholder icon. Serving the SVG as /favicon.ico would work fine.