When you start using an Android device for the first time, you'll frequently be presented with a screen asking you to choose a default application for the action you're trying to perform. For example, click on a link to a web site, and you may be asked which browser you wish to use to open the link:
It rapidly gets annoying to have to respond to this prompt every time you initiate an action, so Android let's you check a box and say "Use this app every time I do this action." That's great, and a real time (and aggravation) saver in the long run. But what do you do when you change your mind. Perhaps you set the built-in browser as the default for certain actions, and now want to set Firefox or Chrome as the default. How do you do that?
There are two methods, one surgically precise, and the other more like a sledge hammer, but sometimes useful.
The precise, and generally preferred way: Go to Settings, open the Application Manager and find the app which is currently selected as the default for that action. In this case, it's Firefox. Scroll down a bit, and find the button labeled "Clear defaults." Tap that, and the next time you click on a link to a web site you'll be prompted again, as above.
The sledge hammer can come in handy if you want to erase ALL your saved default actions or, because of some error condition, you can't figure out which app has been set for a default action. Again, go to Settings and open the Application manager. But instead of selecting a single app, tap the menu button. Then tap the option to "Reset app preferences." You'll be asked if you really want to do this. Tap "Reset apps," and BAM!! Hammer time. All of your default actions are gone. Every time you request an action that can be handled by more than one installed app, Android will once again prompt you for the app you wish to use, and give you the option to set a default.