実はFirefoxの方がシェア下落率は大きい
http://royal.pingdom.com/2012/02/09/google-chrome-could-exceed-50-market-share-by-end-of-2012-study/

>To answer these questions we look a year into the future, to January 2013, and predict how each of
>the browsers will fare. We based this prediction on the average monthly change in market
>share each browser had during January 2011 to January 2012, which was:

+5.08% - Chrome
+2.27% - Safari
-0.13% - Opera
-1.69% - IE
-1.75% - Firefox