This video tells you how to set your default browser to Firefox3.5. In a few simple steps, you can set Firefox as your default browser and also make sure the browser is Firefox whenever you browse. Open Firefox browse- click on Files-Select Tools-Select options-select advanced-you will find System default at the end, just click the option button against". Always check to see if Firefox is the default browser on startup. This option changes your browser to Firefox and also makes sure that always your browser is Firefox.