If you’re a web developer or web designer you’ll need to read this support article I located on Google.

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

Windows or Linux Users

To disable Firebug:

1. Open the Firebug pane in your Gmail tab by clicking the Firebug icon.
2. Click the down arrow next to the Net tab, and select Disable monitor for mail.google.com
3. Repeat Step 2 on the Console and Script tabs.

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.

Here’s the link to the original article. Google support document.