Fiddler can be helpful for:
1. Find files/java scripts that are accessed by the client from the server.
2. When we encounter a strange performance work bottleneck in IE that you can't track down.
3. Task is not executable only on Internet Explorer
4. Able to perform a Specific web activity only on Firefox and not on IE
Fiddler works only with IE
What does Fiddler Fiddle with:
Fiddler records all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data.
Windows 2000 / XP / 2003 / Vista with Microsoft .NET Framework v2.0 +
10 megabytes disk space / 800mhz processor (Screams at 2.4ghz)
128 megabytes RAM (512mb+ highly recommended)
Link to Download Fiddler
A Picture speaks 1000 words: Fiddler Layout
HTTP Statistics view
By exposing all HTTP traffic, Fiddler easily shows which files are used to generate a given page. Using the Statistics page, the user can multiselect to get a "total page weight"—the number of requests and the bytes transferred
Additionally, by exposing HTTP Headers in the Session list, the user can see whether pages are missing HTTP Expiration headers that permit client or proxy caching. If a response does not contain Expires or Cache-Control headers, it might not be cached by the client.
Using Fiddler for Debugging
In addition to seeing all HTTP requests and responses, Fiddler supports the notion of breakpoints. When the Enable Single Step Debugging option is checked on the Rules menu, or when the properties of the HTTP Request or Response match the target criteria, Fiddler can pause HTTP traffic and allow edits. This feature proves useful for security testing, as well as for general functionality testing, because all code paths can be exercised.