I'm a front-end web developper, and since Color Oracle software ceased to function with my setting ("recent Ubuntu versions are not supported"), it was really difficult to test webpages I worked on… So I'm grateful that this add-on exists.

As said by Skylar, there is room for improvement: position:fixed seems not supported, and it would be great to have Tritanopia simulation too. But it's already great as it is - so 5 stars.

Easy to use, does exactly what it claims.