RGBlind is a simple color blindness simulator for the web.
The add-on uses SVG filters to display web pages as they're seen by people affected by two forms or red-green color blindness, protanopia or deuteranopia. This may help you spot visual details they might miss, for example text that becomes hard to read because it blends into the background.
There's no complexity to the interface. Clicking RGBlind's address bar icon displays "Protanopia" and "Deuteranopia" options; selecting one of these updates your screen colors according; clicking "Normal vision" restores the original colors. And that's about it.
If you're designing a website - or any other HTML-based document - it's important to consider how it might be viewed by anyone affected by color blindness. RGBlind only does this in a very basic way for only two red-green color blindness variants, but if you don't have anything similar it's a good place to start.