A simple workaround is to desactivate Add-on signature verification :
- enter the config page of Firefox (in the Location bar) : about:config
- search for : xpinstall.signatures.required
- double-click on the line, it will switch it to "false"
- Your FiddlerHook add-on may now be activated and fully fonctionnal
!! Ensure that you have no unwanted add-on before unchecking signatures ..