Some months back, it was announced here that Firefox 4.0 would use Microsoft's DirectWrite text rendering, meaning, among other things, that CFF OpenType fonts would be rendered using ClearType subpixel rendering, like TTFs, instead of the old, much lamented Windows PostScript rasteriser. 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. He graciously gave his permission for me to post his reply here, and to continue the discussion on this forum.
DirectWrite is on by default but only when hardware acceleration is enabled. There's a long list of whitelisted/blacklisted drivers that we maintain, mainly because of various driver bugs. MS has something similar but I imagine they can support a wider set of drivers because they know more about precisely where drivers have bugs. The latest numbers indicate that over half of Firefox users on Windows 7 run without acceleration enabled. That's based on a combination of underlying hardware and driver versions so those those running without acceleration will probably decrease over time as drivers get updated.
I'm pushing to have DirectWrite enabled *always* on machines that support it (i.e. whether hardware accelaration is enabled or not) but that's been hard to do because there's been a lot of pushback among users who are accustomed to existing GDI rendering. This is especially true for the core web fonts (Arial, Tahoma, Verdana) at 9-13ppem sizes. The default DirectWrite rasterization is relatively light and so there have been a fair number of complaints about fonts being "fuzzy". For now, we've created a list of fonts that default to GDI rendering at small sizes (<16ppem) with fonts like Arial, Tahoma, Verdana on it. I'm hoping we'll be able to sufficiently resolve this for those users to allow us to enable DirectWrite always in a future version.
One immediate thought that I have regarding this is that perhaps Mozilla could find a way to enable DWrite by default for CFF fonts? I don't think anyone is going to complain that this is worse than the old PS rasteriser rendering!