FiddlerHook was deprecated a very long time ago, as Mozilla declined to sign the extension and it's no longer needed anyway. See http://www.telerik.com/blogs/configuring-firefox-for-fiddler for details on how to point Firefox at the system proxy (which it usually is already) and how to trust Fiddler's root certificate (which always takes a manual step. Visiting http://127.0.0.1:8888/fiddlerroot.cer and clicking OK a few times is usually enough).
FiddlerHook was deprecated a very long time ago, as Mozilla declined to sign the extension and it's no longer needed anyway. See http://www.telerik.com/blogs/configuring-firefox-for-fiddler for details on how to point Firefox at the system proxy (which it usually is already) and how to trust Fiddler's root certificate (which always takes a manual step. Visiting http://127.0.0.1:8888/fiddlerroot.cer and clicking OK a few times is usually enough).