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.