This question is pretty old here. Here's an old answer. I've tested this method,and it works very well:

• Open the Android "Settings" application.
• Tap on "Applications" then "Manage Applications."
• Tap on the "All" tab, then find Browser and tap on it.
• Under "Launch by default" press the "Clear Defaults" button.
• Now when you open a link, Android will ask which browser to use, and offer to set a new default-choose Mozilla Firefox.

-Best regards, Osman A.