The article's ADI charts do not account for Mozilla moving Windows XP and Vista users from the Firefox release channel to the ESR (Extended Support Release) channel in March 2017 [1]. New versions of Firefox do not support XP or Vista, but XP and Vista users will continue to receive ESR security updates at least through 2018 Q1. You can see a similar "drop" in Mozilla's Firefox Hardware Report [2].
[1] https://blog.mozilla.org/futurereleases/2016/12/23/firefox-s...
[2] https://hardware.metrics.mozilla.com/