*Adds a plain brightness slider menu popup icon. *No background scripts using up resources. *Few permissions needed. The above two properties mean that page reloads and changes will clear the brightness. However, it can be immediately re-applied by either clicking the icon or by using the shortcut.
Shortcut: ctrl+shift+L (configure in about:addons -> gear icon -> "Manage Extension Shortcuts"). Use the shortcut, press tab to select the slider, then use left/right arrows to adjust brightness, and then escape to hide the slider.
The brightness CSS filter applied to <html> does not currently apply to backgrounds with the "fixed" property; presumed to be a Firefox bug.