The HTTP code is in mozilla/netwerk/protocol/http in the Firefox source
code. If you have any specific questions, you can also ask
mozilla.dev.tech.network. http://www.mozilla.org/projects/netlib/ also
has some documentation.
I'm not sure if that's what you were looking for, but your question was
very general...
-christian
--
All the world's a stage,
And all the men and women merely players:
They have their exits and their entrances;
And one man in his time plays many parts, [...] --W. Shakespeare