Firebug is known to make Gmail slow unless.
26 November 2007

Determine if Firebug running...

So I finally got the new gmail for a few weeks now and I love it. Preloading makes life a little faster … but it warns me about firebug … How did it know?

After some digging it was really quite simple …

if(window.console && window.console.firebug)
{
 alert("you are using firebug");
}

Try it out for yourself by clicking the following link: Is Firebug running?

Some people have a problem with this but it’s a proactive way for them to make their email be more responsive… I can’t live without Firebug, because it’s a great tool, BUT I keep Firebug disabled most of the time because it slows my browsing experience.

Cool !