Wireshark, Firebug, Fiddler all do similar things - capture network traffic.
Wireshark captures any kind of network packet. It can capture packet details below TCP/IP (HTTP is at the top). It does have filters to reduce the noise it captures.
Firebug tracks each request the browser page makes and captures the associated headers and the time taken for each stage of the request (DNS, receiving, sending, ...).
Fiddler works as an HTTP/HTTPS proxy. It captures every HTTP request the computer makes and records everything associated with it. It does allow things like converting post variables to a table form and editing/replaying requests. It doesn't, by default, capture localhost traffic in IE, see the FAQ for the workaround.