Focus (https://heyfocus.com) is a Mac application that blocks distractions like Facebook, Twitter, Reddit, etc...
It works out of the box with Safari, Chrome and Vivaldi, but needs an extension for other browsers like Firefox.
This single extension should work for all of those other browsers.
Download the latest official releases at https://github.com/bradjasper/focus-extension/releases
Most browsers have a way to load an extension from a file. Download the extension and load it into your preferred browser.
If things aren't working, please contact support@heyfocus.com