The regular Firefox browsers have a more complete user agent (example: Mozilla/5.0 (Windows; U; Windows NT 6.0; fr; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1). My regex only matches the first part: Mozilla/5.0. As I know, no browser uses this user agent. Firefox on my Mac still receives the header with the UTF-8 value.