Bug 363276 - Specify more properties so Gecko doesn't guess 'wrong'.

ChatZilla only.
r=samuel

extensions/irc/xul/content/output-window.html

@@ -11,6 +11,8 @@
         .header-outer {
           position: fixed;
           top: 0px;
+          left: 0px;
+          right: 0px;
           margin: 0px;
           padding: 0px;
         }