It's very simple. Just press and hold the Volume Down and Power buttons at the same time. You'll see an animation on the screen, indicating that the screen photo was saved.

That isn't working. I'm using a Samsung Galaxy Lite phone.

You just press and hold the home button and the sleep/power button at the same time for a few seconds until a white frame appears around the edges of the screen , If that sounds too complicated, hold the home and power button at the same time for a few seconds

We can help with Firefox for Android, but we don't always know the ins and outs of system differences on all the Android devices. For that, you could use an Android-enthusiast forum like: http://forums.androidcentral.com/

Thank you