Did you ever reach a website which said something like "Your browser is too old". Sometimes it is not easily possible to upgrade your browser like if you have an Asus Eee PC. Here's how you can change your user agent so that those annoying websites will work again.
With Firefox this is a peace of cake. First install the User Agent Switcher add-on for Firefox. Once you have clicked on "Download Now" and have restarted your Firefox you will have a new menu under Tools (Default User Agent) which lets you change your user agent to anything you want.
Once you have changed your user agent you can verify it on this site.