Like Google Talk, you can also run the Facebook Chat application from Firefox sidebar.
The advantages are that you need not open your Facebook homepage to see a list of friends that are online and second, you can talk with Facebook friends while reading any web page in Firefox.
To run the Facebook chat messenger from Firefox sidebar, press Ctrl+Shift+B to open the Bookmarks Organizer.
Now go to Organize -> New Bookmark and type the following address:
Also tick the check-box the says "Load this bookmark in the sidebar". You may save this under "Bookmarks Toolbar" so you can open Facebook chat window in a single click.
You can do something similar in Opera – Press F4 to open the Opera sidebar and drag the above Facebook link onto that Opera Sidebar.
Related: Add iGoogle to Firefox Sidebar