Wireshark, Firebug, Fiddler all do similar things – capture network traffic.
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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.
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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, …).
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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.