Chatzilla opens in a new window, you should take a quick glance at the preferences before you connect to a server. If you do not want to exchange files you should disable DCC in the options. Supported Networks are Efnet, Dalnet, Quakenet and Undernet plus some additional ones. Clicking on a link of a network connects to one of its servers. It is now time to join channels, do so by clicking on IRC > Join channel.

The channel search looks similar to the one offered by mirc, type in a word (xbox, games, counterstrike) and the channel list will display every channel with such a name or topic. Double-Clicking a channel in that list joins that channel and you see the list of users on the right side and the main channel window with the public chat in the middle. If you right-click a user you can open a private chat or send that user a file for instance.

I like this add-on. I sometimes search for specialized information and find that only on IRC. For example if you need coding help you might want to join a channel like #html or #php. The people there are skilled and willed to help you if you phrase your requests properly.