How does GMail know I use Firebug extension in Iceweasel?

[Raj Kiran Grandhi <grajkiran@gmail.com>]

Recently gmail has begun displaying the following notice when I login to 
its web interface:

"Firebug is known to make Gmail slow unless it is configured correctly."

It offers a couple of option to disable/configure it. However what I 
find disturbing is how gmail 'knows' what extensions I have installed on 
my machine. Is there a bug in iceweasel/firefox that leaks out some 
information on extensions to the remote servers?

A search on google tells me that many others have noticed this too, but 
I could not find how google does it.

Thanks,
Raj Kiran