change FFX to Firefox to be consistent with rest of file

ChangeLog

@@ -348,7 +348,7 @@ train-2011.09.22:
   * improved first-time development experience: `git clone && cd browserid && npm install && npm run`
   * initial support for running locally under virtualbox via vagrant: #261 (thanks ozten!)
   * (fix 2011.09.23) fix race condition between relay iframe and window introduced with IE9 support: #287
-  * (fix 2011.09.23) fix blank popup on second signin invocation in same session in FFX: #286
+  * (fix 2011.09.23) fix blank popup on second signin invocation in same session in Firefox: #286
   * (fix 2011.09.23) explicitly disable caching for /wsapi calls, prevents unwanted caching of CSRF and friends: #294
 
 train-2011.09.01: