Okay, so I did a few tests:
1) Opened a new instance of Firefox (no others were running) and loaded the page. The page loads, I open Firebug to see the console and nothing is in there:

However, there is stuff in the other Firebug tabs (see the cookie tab below):

2) I close Firebug down, refresh the page and open it up again and that's when I get the double message:

3) When I minimize the Firebug window (to its home on Firefox's status bar) and refresh the page four times in a row, then open it again, I get 8 messages! Which makes sense I guess:

It's a fresh "installation" of Code Igniter (1.7.1) and a fresh install of the FirePHP addon to Firefox 3.0.7. I then added the FireIgnited plugin in to CI (and also this "FireIgnited extended plugin":
http://codeigniter.com/wiki/FirePHP_4_CI/).
The code you saw in my previous post is the only place I've added a call to FirePHP.
Any ideas?