It kinds of depends on how you look at things. If you go on browser versions alone, then IE is actually the #1 browser (as this article ridiculously points out).
But if you look at things realistically, combining all versions, IE is indeed ahead of Firefox:
But again, some might not think that’s realistic either, so they might prefer to combine Chrome and Firefox, but not IE:
There’s no perfect way to determine “most popular browser” but I think it’s safest to say the most popular, in order, are Chrome, IE, then Firefox.