Firebug clashes with Gmail
Today Gmail made a note in my account page.
Firebug is known to make Gmail slow unless it is configured correctly. Fix this
To disable Firebug:
Click the green or red icon in the bottom right corner of the browser window to open Firebug.
Click the bug icon in the top left corner of Firebug and select ‘Disable Firebug for mail.google.com.’
If you are not willing or cannot disable your Firebug ,in that case there is another solution.
Open Firebug and in the Console tab select Options.
Uncheck ‘Show XMLHttpRequests’.
Also click the ‘Net’ tab select Options and check ‘Disable Network Monitoring.’
Get Firebug :: Firefox Add-ons here