Currently, Apple does not allow you to change the default browser on iPad, iPhone and iPod touch devices.
You can, however, send pages from Safari to Firefox by tapping the share icon 
and choosing Firefox as the destination. If Firefox is not listed in the options, tap the More button (three dots) and tap the switch next to Firefox to enable it.
If you would like to see this change in the future, please leave feedback for Apple on their site: http://www.apple.com/feedback/.
