Built in debugging in IE 8 and 9
F12 in IE 8 and 9
For Java script debugging and built in network monitor in IE9 press F12
Enable Script Debugging first.
Before debugging, open an IE browser and make sure in:
Tools->Internet Options->Advanced->Browsing
Disable script debugging (Internet Explorer) is unchecked
IE 8 JavaScript Debugger
https://msdn.microsoft.com/en-us/ie/dd699458
IE 8 JavaScript Profiler
https://msdn.microsoft.com/en-gb/ie/dd490714
IE 9 Developer Tools
https://msdn.microsoft.com/en-gb/ie/hh545573
Built in network monitor skip to 25 minutes
Best Regards
Dynamics CRM Team