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XmlHttpRequestDebugging
Allows you to debug XmlHttpRequest calls with an in-browser UI
Creates a floating console where you can see and manipulate XmlHttpRequest calls.
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Now also exists for IE:
http://blog.monstuff.com/archives/000291.html
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Version 1.2 is now available. Fixes an issue with Firefox 1.5 and Greasemonkey 0.6.4.
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Version 1.1 is now available. Fixes an issue with onreadystate callbacks.