If you press Ctrl->Shift->m when viewing a website, it will be switched to a mobile view that allows you to see what the website looks like at various mobile resolutions. I found this out by accident. But this is a very cool Firefox trick. Another reason to not use Chrome. I will never use that browser again. I only use Firefox. This would be best used in conjunction with a user agent changer to emulate various mobile devices. This makes testing a website for mobile customers very easy. The coolest trick in Firefox yet.
There is a list of Firefox keyboard shortcuts here, but I could not find this one there: http://www.shortcutworld.com/en/win/Firefox.html.