Don't build p3p for Firefox, we don't have any UI for it.

browser/config/mozconfig

@@ -8,7 +8,7 @@ mk_add_options MOZ_PHOENIX=1
 
 ac_add_options --disable-ldap
 ac_add_options --disable-mailnews
-ac_add_options --enable-extensions=cookie,xml-rpc,xmlextras,p3p,pref,transformiix,universalchardet,typeaheadfind,webservices,inspector
+ac_add_options --enable-extensions=cookie,xml-rpc,xmlextras,pref,transformiix,universalchardet,typeaheadfind,webservices,inspector
 ac_add_options --enable-crypto
 ac_add_options --disable-composer
 ac_add_options --disable-profilesharing