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

Suggestion from Google

For the best Gmail performance, we suggest disabling Firebug for www.google.com.

You can find instructions for the most recent version of Firebug at the Firebug documentation site.

If disabling Firebug for Gmail doesn’t improve performance results, you may have to entirely disable Firebug.

http://mail.google.com/support/bin/answer.py?hl=en&ctx=mail&answer=77355