We know, we know. It seems like just yesterday that Firefox 4 was the hot new shit on the block. But since then Mozilla has adopted a new "fast release" schedule in order to stay ahead in the browser wars. That means lots more updates compiled into "stable" releases and shipped to users like you.
Get the latest and greatest Firefox, now faster than ever thanks to a memory management overhaul, over at Mozilla now. Or, your browser should automatically update as usual in the next day or two.