There's a good article on rapid web development using Firefox posted on the Vertigo Software Blogs. I especially liked the slides linked at the bottom of the post. I've installed all plugins that appeared useful in the first place.
I've been using the developer tools (DOM Inspector, JavaScript Console) included in the Firefox redist for some years now and have always been blown away by their usefulness. Currently there are neither open tags to close nor quirks to eliminate for me, set aside that there have been some annoyances with the layout of this page. Let me see how much additional value the tools bring when I encounter the next problem. According to Murphy's Laws I will!
Unfortunately there's nothing that flexible and powerful for Internet Explorer. The IE DevToolbar tries to mimic the functionality of the Firefox DOM Inspector, but is far from being perfect. Nevertheless, better that nothing.
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