Here's a tiny firefox extension that blocks web pages by adding friction. When a site is blocked, you have to make a conscious decision to override the landing page by typing "I NEED ". This unblocks the site for 1 minute. You can remove a site from the block list by clicking "Edit", but this option is unavailable from the blocked landing page.
There are plenty of extensions that do this same thing, but there's a lot of value in adding a workflow that works exactly as I want. Also, I'm not interested in paying for something this simple.