Nice and simple. Just works. The list of sites to block, reached by editing the add-on's preferences, is crowded like the 1990s Yahoo.com homepage and makes my eyes want to skip over it, so i have to sort of force myself to recall that the place to add new sites to block is just above that, but other than that cosmetic quibble this extension simply does exactly as it says — blocks sites — without the temptation of setting times that sites are available which leads to wasting time tweaking settings.