This is great news. Alexa rankings, though seriously flawed, serve as a way to rank websites. They use a toolbar that is installed in the browser to let them know approximate relative traffic levels for every website on the internet. This ranking is used by several companies to assess the value of a given website, and thus put determine an appropriate price for advertising on the site.
Until this week the Alexa toolbar was only available for Internet Explorer. Since my site receives a disproportionate number of Firefox users (34%) my ranking is probably not as high as it should be.