John Daggett: "In Firefox 6 we’ve added the ability to alter prefs that affect the underlying ClearType subpixel anti-aliasing that is used. [...] Because of the large differences between GDI and DirectWrite renderings for some fonts at small text sizes, for Firefox 7 we’ve implemented a pref that specifies a list of fonts for which GDI rendering will be used at sizes below 16px. By default the list contains fonts such as Arial, Tahoma, Verdana, Trebuchet MS, Segoe UI and Consolas. Downloadable fonts will always use DirectWrite."

John Hudson: "When Firefox 4.0 arrived, I was surprised to find that DWrite rendering was not active by default, and that I had to manually set some config flags in order to activate it (not something I'd be comfortable recommending to the average browser user). This week I wrote to John Daggett at Mozilla, and asked him to comment on this and whether there is any change in Firefox 5.0."