Sniff Network Traffic
Sniffing Unencrypted Traffic
tcpdump -i eth0
Sniffing HTTPS traffic
Initiate the connection yourself
openssl s_client -connect myserver.example.org:443
Configure Wireshark to decrypt
This option requires the private key of the server you're connecting with.
(Check if wireshark is configured with GnuTLS: "wireshark -v" should list "with GnuTLS" and "with Gcrypt", otherwise, the option to add a private RSA key is not visible in the preferences)
If you prefer a command line alternative over wireshark, check out ssldump.
Use a proxy server
This option alters the certificate of the server, so your webbrowser be be configured to accept the new certificate.
(Charles is a paid application.)
I expected a Firefox extension that could show the decrypted HTTPS traffic, but so far I have not found anything like this.