While testing the Firefox Nightly (v16) Beta I learned you can test websites for responsiveness (aka mobile and tablet compatibility) from within Firefox. Until now I mostly used the hosted QuirkTools Screenfly application.
You can view a specific site in variety of views that cover the most common mobile, tablet and notebook screen display sizes. You can also rotate the display to landscape view (supported by several mobile/tablet devices). You can also set a custom size by dragging the bottom right window edge.
To access this handy Firefox feature, press Alt key to display menu toolbar, then click Tools > Web Developer > Responsive Design View. Or use the Ctrl + Shift + M shortcut.
The Responsive View option remains specific to the Tab you invoked it in and doesn’t affect other open tabs.To switch back to regular view just use the Ctrl + Shift + M shortcut again.