The power settings are controlled by the Windows operating system, not Firefox. The [https://www.dell.com/support/article/en-ca/sln152096/how-to-configure-the-windows-8-power-plan-and-settings-tool-on-your-dell-pc How to configure the Windows 8 Power Plan and Settings tool on your Dell PC] page has some detailed steps on how to adjust these settings. It's a Dell support page, but the steps should apply to any computer running Windows 8 or Windows 8.1. Hope this helps.