No idea about the gestures in firefox, but in the default browser you need to slide your finger/stylus in from outside the left side of the screen. So for example put your finger below the speaker and then slide it right until the arrow appears.. and be quick to tap it, the arrow disappears quite quick.

Zooming is quite easy to do with stylus, just draw a circle on the screen like you would with a pen. You don't need to try and make it spiral or anything, a normal circle is fine, and the size doesn't really matter either. Once you get it right with stylus you can adapt it to your finger. What caused me problems during the first days was that due to the touch screen being resistive I found that I needed to press the screen quite a bit when doing the circular motion for it to be detected correctly.