[Update 2014-01-16: A point of clarification. There are two possible ways to send a password for IRC. One is supported in the Instantbird UI – it’s the one that automatically identifies your nick with NickServ, the bot which makes sure people don’t steal other people’s nicks. The other, which is rarer but which I needed, involves sending a password to connect at all, using the PASS command in the IRC protocol. That is what is documented here.]

I was trying to do this; turns out it currently requires about:config manipulation and is not documented anywhere I can find.

Using about:config (type /about config in a message window, or access via Preferences), set the following prefs:

messenger.account.accountN.options.serverPassword
messenger.account.accountN.options.username

to the obvious values. Other useful tip: if the IRC server uses a self-signed cert, connect to it on the right port using Firefox and HTTPS, and you can save the cert out of the warning/exception dialog you get. You can then import it into Instantbird using the deeply-buried Certificate section of the Advanced Preferences and it will trust the cert and connect. (I think this is what I did, although memory is hazy.)

http://feedproxy.google.com/~r/HackingForChrist/~3/TOpayXm1S4Q/