With so many different screens resolutions on the market, from super wide monitors to Retina-display iPads to low-res 800×600 resolutions for people who prefer seeing everything big, most web developers will need to know how their websites look in different resolutions. One easy way to do this is using a Firefox plug-in called Web Developer. It has many features including the ability to turn on and off CSS and Javascript, will validate your page.
For more information, visit https://addons.mozilla.org/en-US/firefox/addon/web-developer/?src=ss