At the time of Acid3's release, Mozilla Firefox developers had been preparing for the imminent release of Firefox 3, focusing more on stability than Acid3 success.
Consequently, Firefox 3 had a score of 71.
Firefox 3.5 scored 93/100, and Firefox 3.6 scored 94/100.
Initially, Firefox 4 scored 97/100, because it did not support SVG fonts.
Later, Firefox 4 scored 100/100, because the SVG font tests were removed from Acid3.

According to Mozilla employee Robert O'Callahan, Firefox did not support SVG fonts because Mozilla considered WOFF a superior alternative to SVG fonts.
Another Mozilla engineer, Boris Zbarsky, claimed that the subset of the specification implemented in Webkit and Opera gives no benefits to web authors or users over WOFF, and he asserted that implementing SVG Fonts fully in a web browser is hard because it was "not designed with integration with HTML in mind".

On 2 April 2010, Ian Hickson made minor changes to the test after Mozilla, due to privacy concerns, altered the way Gecko handles the :visited pseudo-class.

Firefox 51.0a1 made a regression from 100 to 99 on 14 September 2016; Firefox 55.0a1 further regressed to 97 on 1 May 2017.
In Firefox Quantum versions, 63.0 received 97/100; 64.0 got 96/100, 68.1.0esr got 97/100.
At the time of Acid3's release, Mozilla Firefox developers had been preparing for the imminent release of Firefox 3, focusing more on stability than Acid3 success. Consequently, Firefox 3 had a score of 71. Firefox 3.5 scored 93/100, and Firefox 3.6 scored 94/100. Initially, Firefox 4 scored 97/100, because it did not support SVG fonts. Later, Firefox 4 scored 100/100, because the SVG font tests were removed from Acid3.
According to Mozilla employee Robert O'Callahan, Firefox did not support SVG fonts because Mozilla considered WOFF a superior alternative to SVG fonts. Another Mozilla engineer, Boris Zbarsky, claimed that the subset of the specification implemented in Webkit and Opera gives no benefits to web authors or users over WOFF, and he asserted that implementing SVG Fonts fully in a web browser is hard because it was "not designed with integration with HTML in mind".
On 2 April 2010, Ian Hickson made minor changes to the test after Mozilla, due to privacy concerns, altered the way Gecko handles the :visited pseudo-class.
Firefox 51.0a1 made a regression from 100 to 99 on 14 September 2016; Firefox 55.0a1 further regressed to 97 on 1 May 2017. In Firefox Quantum versions, 63.0 received 97/100; 64.0 got 96/100, 68.1.0esr got 97/100.
Acid3