As it turns out, this isn't the final 1.0 version. PR stands for "Preview Release," which Netscape used to do once upon a time. The full user strong is"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.3) Gecko/20040903 Firefox/1.0 PR (NOT FINAL)."
But it rocks, of course
. Seems even more responsive than earlier builds.