“Fx” is and basically has always been (v1.5) the official abbreviation for “Firefox”. This abbreviation is basically the only one used inside Mozilla & you can see it reflected in their project names to this day.
> How do I capitalize Firefox? How do I abbreviate it?
> Only the first letter is capitalized (so it's Firefox, not FireFox.) The preferred abbreviation is "Fx" or "fx".
> How do I capitalize Firefox? How do I abbreviate it?
> Only the first letter is capitalized (so it's Firefox, not FireFox.) The preferred abbreviation is "Fx" or "fx".
– Mozilla Firefox 1.5 Release Notes, https://website-archive.mozilla.org/www.mozilla.org/firefox_...