Firefox despite being a popular browser, currently has no version for mobile devices. Mobilify is a Firefox extension which is designed to show a compact view of the content size of webpages, in order to simulate the Firefox Mobile UI. It adds a button on the Add-on bar (formerly known as status bar) in Firefox 4 which enables instantly viewing the webpage in a mobile like user interface. The surroundings of the webpage are darkened and the content is visible within a mobile phone frame. To revert to normal view, click on the add-on bar icon again.
Mobilify is quite an out of the box extension which provides the utility for developers to narrow the <body> to 320px, with a darkening effect for the surrounding of the mobile frame that displays the browser content. This handy simulation can provide an effective overview within the Firefox browser window for determining the visual requirements of mobile users in relation to webpages, web apps, mobile games and applications, etc.
Post Update: Mobilify Extension for Firefox is no longer available on the original destination link. You can download this extension from the alternative mirror link given below.