How do I fix GitHub icons (octicons) on Firefox?

I had the same problem until I whitelisted fastly.net in NoScript. If you are using NoScript or a similar browser extension that can block third party websites, be sure to enable fastly.net because GitHub seems to have started to use this domain to distribute various assets including CSS and JavaScript code as well as some logos and perhaps the Octicons font they use for icons.

Go into Firefox / Preferences / Content / Fonts & Colors / Advanced... and check Allow pages to choose their own fonts, instead of my selections above.

I didn't even remember that I had this setting unchecked. I remember doing it because Google search results would always be italicized or something like that.