I also like is its inspecting html and modifying html and css on fly and see the results with helpful tooltips. It also help in positioning of divs, images with its nice feature of visuallizing css metrics with ruler and box model coloring(HTML inspecting).
It other great feature is its interface which is comes with browser window split in two halves one for your website and one is extension interface (though it also can be detached from it). I am also a fan of Web developer toolbar extension but what it lacks is a java debugger and this great interface but both are a must of a web developer.
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