This extension is required in order to make Oh no! Reflow! work properly. It exposes an experimental WebExtension API that allows Oh no! Reflow! to add a listener for uninterruptible reflow occurring in each browser window.
See the README.md file for more details.
Note: this is an unsigned add-on that will only work in pre-release builds of Firefox (Nightly, Developer Edition). You may need to set xpinstall.signatures.required to false in about:config.