I was looking for a blocker that would replace the Go Fucking Work extension I’m using, as I wanted to switch to Firefox from Chrome. Go Fucking Work extension simply blocks websites to prevent you from visiting distracting websites such as Facebook, Twitter and YouTube. You can also set break times for certain periods of the day, or you can immediately have a 5, 10, 20 etc minute breaks.
I found a couple of similar extensions in the addons.mozilla.org/ however they were not so simple to use and used older add-on APIs, so they would not work with Firefox 57 and up. So I decided to build myself an extension. At this point this extension only blocks the websites you provide and allows you to turn blocker on of off from the toolbar. There are no fancy features such as setting 10 minute breaks and having predetermined schedules for break times. So there there is room for improvement feature-wise.
It uses WebExtensions API that will be the only way of building extension on Firefox in a year. So this extension will last beyond Firefox 57 where the extensions using older APIs will stop working. Even though WebExtension API is cross-browser API, you still have to pay a little bit attention to make it work on Chrome and Firefox at the same time. I have failed to do so.
Please refer to this guide about how to use the extension.