W3Schools currently shows Firefox with 24.5% (and 25.1% record peak).
http://www.w3schools.com/browsers/browsers_stats.asp
Yeah, but I don't buy that, really. Web developers are more likely to have adopted FF than the public at large, if for no other reason than for compatibility testing, and that's the target audience for W3. I just checked the March logs for one of my (very non-technical) sites, and FF is right around 10% - FF/Mozilla-compatible combined is around 14%.