Mozilla's Firefox browser definitely doesn't have the popularity it once did. According to Adobe, Firefox was the second-leading browser behind Internet Explorer in U.S. desktop and mobile usage combined until around the end of 2011. At that point, Firefox was eclipsed by current leader Chrome, as well as Safari and Android browsers.
Experts attribute Firefox's downturn to the fact that they've been notoriously absent in the mobile sphere. However, Mozilla does seem to be coming up with ways to solidify their small but dedicated user base, such as the new Firefox Developer Edition.