Fine-tuning touch viewport works on Chrome/Android using
<?page viewport="width=device-width, initial-scale=0.9, minimum-scale=0.5, maximum-scale=4.0, user-scalable=yes"?>
while it does not on Firefox/Android. However, as a workaround
<?meta name="viewport" content="width=device-width, initial-scale=0.9, minimum-scale=0.5, maximum-scale=4.0, user-scalable=yes"?>
works for both and does not interfere with desktop browsers AFAIK.