component/firefox: Don't discard firefox's stderr

This reverts 8f35ea4d
It should no longer be needed because erp5.util should now have the fix
from
nexedi/erp5@97f3310e

component/firefox/buildout.cfg

@@ -40,7 +40,7 @@ script =
   cd {}
   export LD_LIBRARY_PATH=$PWD:{}
   export PATH={}:$PATH
-  exec ./firefox "$@" 2>/dev/null
+  exec ./firefox "$@"
   """.format(
     firefox,
     ':'.join(part['library'].split()),