I was quiet thrilled when Gmail showed me this error msg. Gmail detected the browser to be firefox and the firebug addon turned on. I use firebug extension when I make AJAX calls on my projects, it is turned on by default.
In fact Gmail suggest what you can do to fix the problem, here is the link which tells you how to turn the firebug extension disabled for a particular site or even the entire firebug extension to be disabled temporarily.
What else more you need from Gmail.
Well raises more suspicion, which I had already talked about right here, is Google sneaking in to our privacy.