Wow. I just saw in gmail while using Firefox (2) that it knew firebug was running (or maybe installed).
How does gmail do it?
Anyway, the warning was the following in white text on a red background. I kept the link so you can see the explanation and options (basically turn off certain settings in firebug).
Firebug is known to make Gmail slow unless it is configured correctly. Fix this Hide
I'm not alone of course, nor am I particularly timely in seeing this.