Anyone know of a good http header viewer for Firefox?

I've seen many http header viewing plugins for IE, but was wondering if anyone knows of a good one for Firefox, or Opera. If not, then would someone please recommend one for IE? I've seen some for https, cookies, etc, but I just need http for now. I view cookies with JavaScript, and don't currently test https connections. I'm just interested in the headers my site is sending.

I just found addons.mozilla.org/firefox/addon/3829 which I will probably try. I'm still interested if anyone has suggestions.

 

Try Fiddler.

 

Thanks. I think I'll use Web Developer because it includes other features I could use. That is https://addons.mozilla.org/en-US/firefox/addon/60
Fiddler is an interesting tool, and I may use it in the future, but I was looking for a plugin this time.
Thanks again for the recommendations.