install the extension via double click on the LoadFirebug.qar file. You can load the Firebug document extension then into a QlikView app (QVW) via menu Document Properties/Extensions. It will be visible in WebView mode at the buttom of the sheet. Sometimes it doesn't show up the first time so press F5 to refresh.
Btw. I will hold an 2.5 hrs online training "Introduction to QlikView 11 extensions" with a lot of code examples (also theory ;-)) at the QlikOn training center on September 18th. You will find more in their website soon...