A firefox plugin has been created by Meebo (my persoanl all in one online IM client) so that you can have it on your sidebar so you won’t miss a single message when you are browsing with Firefox. They even created a nice post with comics to introduce the debut.
Personally, I find the plugin not so useful. We are able to run Meebo on any browser now and not tied down to Firefox, right? So why the need for a plugin to show it next to your sidebar in Firefox? In any browser, I could simply press CTRL+TAB to switch among the browser tabs to reach the one with Meebo. The tab will even be highlighted when a new message has been received. It seems to me it was more of like a good programming practice in XUL, and good reputation boost to have your application written to be accessibe by a firefox plugin.
Well, some people might still find it useful to have it visible all the time instead of hiding it among a sea of tabs.