If you want to develop websites you’ll need to install all the browser, my setup is:
Google Chrome (default – with developer tools)
Firefox (with deveolper tools)
and for mobiles I use my iphone 4 with chrome/firerfox to test for phones and http://www.browserstack.com/ to test on other devices (it’s not free but you can register as many times as you want with fake emails..I think they did it on purpose) . If you’re on a mac you can use XCode to test on iDevices.