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

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

Windows or Linux Users

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 Gmail 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. Uncheck Show XMLHttpRequests.
5. Click the Net tab.
6. Select Options.
7. Check Disable Network Monitoring.

Mac Users

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

Source: mail.google.com/support

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Rupom