Responsive Design Mode in Firefox

In Firefox 16 (may be in older version) you can switch your page to "Reponsive Design mode" with Cmd + Alt + M (or Ctrl + Shift + M for windows users). In this mode you can adapt your web page to a certain dimension, in the aim to try the display of it with another dimension

You can also try Cmd + Alt + S or Cmd + Alt + I ;)