The first extension that must be installed in any Firefox out-of-the-box, as it can block some unwanted/unneeded access by other extensions. This is the only blocker that actually works the way it should: it allows not only blocking referenced objects (images, scripts, whatnot), but also Pages directly opened (even typing the URL/URI on the address bar). As mentioned, it can also block access by other extensions. Hint: to completely block a site, an entry must be entered twice on the personal list, the second time with a "$document" at the end.

Firefox + uBlock is essential IMO as the first layer of protection in online identity theft, as well as preventing exposing family to inappropriate adds.