When I was testing a site in Firefox, I noticed a couple of buttons to the top right of Firefox’s built in Web Developer Tools. One is called ‘Responsive Design Mode’. You can access this mode either by pressing OPTION-COMMAND-M on your Mac or by going to Tools > Web Developer > Responsive Design View. Now for a little preview of what it can do:
You can view your site as it would appear on a phone by changing the screen size to 320 x 480. You can also use the rotate button to see how the device would react to being rotated. You can even add your own presets, or watch the screen as you make it larger or smaller. Great tool!