After installing Firefox on Android, it will appear in your apps, which may be accessed using a button on the bottom or side of the home screen (depending on how your device is set up). To add a shortcut for Firefox to your home screen, typically you would long-press a blank area of the screen and then choose either Shortcut or App from the menu that pops up. If you want to switch your default browser from the stock Android browser to Firefox, you need to first clear the indication that the other browser is the default. Typically you would do that in the Application settings under Manage Applications. If you can indicate what device it is, a volunteer with a similar device probably could give more specific steps.