One possible solution is to make a little script to open the irc chanel in an existing xchat, and pass that to Firefox.

For example - make a file in /usr/bin called xchat-firefox and put in it:

#!/bin/bash

xchat --existing --url=$@

Add executable permission to the script:

sudo chmod +x xchat-firefox

Now using the menu option Edit-Preferences-Applications Tab in Firefox select "xchat-firefox" for the "irc" content-type.

source 1 & source 2