Web Developer Toolbar
The Web Developer extension adds various web developer tools to a browser. The extension is available for Firefox and Chrome, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.
While it isn’t a mobile-specific extension, it is still an excellent tool to have in your toolbox. It lets you do quite a few different things, such as viewing information about the current page, switching various DOM elements on and off, and using the page size indicator.