I'm looking for an ie8-addon that displays the http headers like firebug or httpliveheaders do. Any advice is appreciated.
edit: I may be blind but it seems as if the built-in developer tools (F12) do not show the http headers.
You might be looking for Fiddler2
Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.
Fiddler is freeware and can debug traffic from virtually any application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more.