It doesn’t look good for Firefox: Almost every month for the last three years, Firefox has lost ground to Internet Explorer, Chrome, and Safari. For most of 2009 the trend was fairly straight as it fended off Chrome and nibbled away at IE, but between 2010 and today Firefox has lost a third of its market share, from a worldwide peak of around 30% down to 20%.
You can look at this two ways. First, the total number of people on the internet is growing, so while Firefox’s share has decreased, the total number of people using Firefox is increasing. The other point of view is that Firefox, whether you like it or not, is declining in popularity.