FiddlerHook is a Firefox plug-in that enables you to use Fiddler with Firefox. You will first need to enable FiddlerHook in Firefox (menu Tools –> Monitor With Fiddler –> Use Fiddler Automatically). Then add a Fiddler button to the navigation bar by right-clicking and selecting Customize. Drag the Fiddler button to the navigation bar, click done and click on the Fiddler button.
HttpFox is similar to Fiddler, and is a “native” Firefox plug-in.
An interesting advantage of using Fiddler through Fiddlerhook with Firefox when compared with Fiddler with Internet Explorer is that you can watch traffic to localhost.
PS: you can use Fiddler in IE if you set Fiddler as proxy for IE.