Easy to use add-on which really helps to stay focused while working. It has two modes: one disabling access to blacklisted websites and other enabling access to only whitelisted ones. I really like the minimal approach enabling you to use it without any additional stuff like visual customization and other unnecessary options. Does what should do really well!

Does what you think it does: overlays the website with a blocker, has an on-off-switch, blacklist and whitelist mode. It easy to add URLs