Leechblock NG is a very useful anti-procrastination tool. Very customizable but yet simple to use.
I've found it useful for blocking websites (e.g. twitter) while keeping certain sub-pages accessible (e.g. my twitter profile page).
One good thing about leechblock is that it works on both Firefox and Chrome, so it's less easy to procrastinate by switching to another browser.

It works great

Leechblock has saved me so much time and energy I used to waste mindlessly surfing and scrolling and watching.

Thank you for the great service, James.
The extension works just as described and has saved me a lot of time.
I really like the timer at the corner of the screen; non-distracting and informative.

I will certainly keep using this add-on.

Simple and easy to use, and with enough features to be super helpful.

Works exactly as intended. I was always skeptical of an extension like this but it really does improve productivity. Only been using it a week and it's already rewiring that part of my brain that would instinctively open new tabs and head for reddit during work hours.

This is one of the best site blocking apps for Firefox. I can't resist browsing news sites at work. The sites were leeching my time.
With this app installed, I can work with full focus. Thanks Devs.

This is really really really good

Improves my productivity at work.

can block keyword... this is a 100% useful feature

Really really good for productivity. It makes it so easy to stop habitually checking a website.

Does what it says. Solid.

Does what it says it does on the box. I really like the customizeability, and the delay pages are great for giving me those two minutes to think about if I really want to go somewhere.