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CloseReview for Live HTTP Headers by blseal
Rated 5 out of 5 stars
Spyder, Lucian, sounds like you're looking for Server Spy (https://addons.mozilla.org/en-US/firefox/addon/2036). Nice little addon, displays the web server type in the status bar, and double-clicking on it displays additional details (like what extensions have been loaded on it).
As for LiveHTTPHeaders, it's the best addon of its type!
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