To change your default browser, first "Clear defaults" for your current default browser. Let's say that the stock Android browser is your default. From the home screen: Menu > Settings > Applications > Manage Applications > All Scroll down to Internet, tap that, scroll down to Launch by default and tap Clear defaults. You're done and you can back out. The next time you hit a link, you should be presented with a choice of browsers. Now you can check the "use by default" box and tap Firefox. Did that solve it?