This fixes Mozilla bug#285267 - navigator.language js property doesn't reflect 
browser default language. It appears that this patch was previously 
proposed upstream but not accepted. (https://bugzilla.mozilla.org/show_bug.cgi?id=285267) 

diff --git a/browser/locales/en-US/firefox-l10n.js b/browser/locales/en-US/firefox-l10n.js
index e9027aa..10bdcdb 100644
--- a/browser/locales/en-US/firefox-l10n.js
+++ b/browser/locales/en-US/firefox-l10n.js
@@ -36,4 +36,4 @@
 
 #filter substitution
 
-pref("general.useragent.locale", "@AB_CD@");
+pref("general.useragent.locale", "chrome://global/locale/intl.properties");