How does this compare to Firebug's similar feature?

Unlike Firebug and TamperData HttpFox does not change behavior of the browser cache, and correctly reports what is being served from cache and what goes on the wire. Unlike TD and LiveHTTPHeaders it has a convenient user interface for examining HTTP conversation post-factum, including timing, cache info, etc. The interface part follows from it being a direct, if not yet complete clone of HttpWatch, an excellent commercial product for IE. Unlike tcpdump/tcpflow/wireshark it allows convenient capture of SSL traffic, and is quick to use.

In summary, HttpFox is the a great tool for HTTP optimization under Firefox.

the Tamper Data add-on is quite similar but also allows you to interrupt/drop requests and change header values and POST parameters on the fly.

Any chance Tamper Data catches an outbound request early enough you can actually rewrite the target URL, including target host?

Because I've wanted to make an extension that does that for a while, but haven't found in Mozilla docs the right hooks, and would love a working model.

You can change any header or content to be sent, including the host header. Is this what you need?

I need to change the actual IP address contacted, and URL (path) requested.

When I just want to see the requests sent to a website, this works fine for me:

tcpdump host thedomain -A -s10000 | grep -Eo "(GET|POST).*$"

More info here:

http://en.gibney.org/capture_webtraffic/

-s0 does what you're trying to do with -s10000

Another alternative to look at is Live HTTP Headers: https://addons.mozilla.org/en-US/firefox/addon/3829

If you work in an IE world, Fiddler is a nice tool.

Now why would someone want to do that :P