Mozilla's browsers global usage share is 7.35 percent according to OneStat.com
And it was just released a few weeks ago into a market dominated by a vendor who has been making browsers since around 1995. This statistic also makes Mozilla's Firefox the number 4 browser globally, with only various IE browser types being more used. Pay attention web designers! You'll not only need to understand the CSS box model, but also why MS browsers calculate width differently than most other browsers.
Firefox is more responsive to the needs of the web community than other browser vendors have been. Today's example: Type sizes are no longer fixed. It has been possible for some time to specify a fixed type size, such as "12 pixels" for the display of type in a browser window. Firefox allows the user to scale this type to the size that is best for the user, not the designer. (Yup, we're talking about you, you 9 pixel font using, two sandwitch eating... oops that's another post.)
Choice is good. It will make the other browser vendors take notice and try to catch up. (Or not... who cares as long as Firefox keeps being so darn good.)