Scared of what your browser might pull up? Cover will protect you from shocking images. All pictures are blurred and you can make them fully visible with a mouse move, one by one.
Here is a little more about the add-on from the developer:
This add-on covers all images with a partially transparent layer. Now decide which images are safe and uncover them one by one with a mouse move. This protects you from shocking and flashing images. There is no protection within the first few seconds of a new page loading, depending, on how long the first images of the website take to load. Uncovered images get blurred again, when you click on the cover button, or when the website dynamically changes.
I’m not usually one to censor things but this add-on comes in handy to cover things you might not want to see. Check out Cover on the Firefox Add-ons website.