PAW will invert all images and videos in webpages so you can use your OS in negate mode and save your eyes.
This addon is useless if you don't use negate mode system-wide. On Mac OS, you can enable "Invert Colors" in System Preferences > Accessibility.
At the moment it's pretty basic, but enough for my every day use.
- Toggle switch as a userbar button to enable/disable the addon globally
You can install the published version from the Firefox Add-ons site