This extension works great, only issue in Firefox Developer Edition is if you end up going to the webpage which you have blocked, instead of showing "Sorry requested cannot be viewed" it is stuck on a refresh loop for a good 5 mins before it gets there. Sometimes this issue does not occur at all. I would also like to mention that if I close the tab that is stuck on a redirect loop, the tab I switched to is now also infected with this redirect loop. The extension insists on trying to load the "Cannot be viewed" message page but I don't understand why it goes into a redirect loop.