If you are browsing gmail on firefox with firebug(error debugger firefox addon) GMAIL states a line at top “Firebug is known to make Gmail slow unless it is configured correctly.”

Its strange !! Anyhow for the best Google Mail performance, google suggest disabling Firebug for www.google.com.

To disable Firebug:
1. Click the green or red icon in the bottom right corner of the browser window to open Firebug.
2. Click the bug icon in the top left corner of Firebug and select ‘Disable Firebug for mail.google.com’.

If you’d like to keep Firebug running, you may improve Google Mail performance by following these steps:
1. Click the green or red icon in the bottom right corner of the browser window to open Firebug.
2. Click the Console tab.
3. Select Options.
4. Un-tick Show XMLHttpRequests.
5. Click the Net tab.
6. Select Options.
7. Tick Disable Network Monitoring.