Great functionality, works exactly like the Chrome extension, giving fully-sighted people a feeling for potential issues with web pages for people with vision deficiencies of several kinds.
There are some small issues: On the latest versions of Firefox, the panel that appears for the extension's options is cut off. There is also an issue where certain vision filters (like blur) only apply to a small portion of the screen. Viewing an image by itself with one of the color deficiency options causes the image to shift position, sometimes off the screen.
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.
And it's very comprehensive: proto/deuterano/tritanopia & -anomalies, cataract, nystagmus, low contrast…