Revert Site lets you get the old Facebook design / layout back. It makes Facebook think you are using an old out of date browser, so it shows you the old design instead of the new one, and also prevents Facebook from switching you back to the new design (until they completely remove the old design, which may be a few months time or longer)
Revert Site is very quick to install, and very easy to use, it just has a single option, that is turned on by default, so you can just install it and not have to configure anything. However if you wish to temporarily disable Revert Site, you can easily do so by clicking the extension icon, in the browser's extensions toolbar, then un-ticking the "Switch back to the old Facebook design" checkbox. To turn it back on, just re-tick the checkbox.
If you can't find the Revert Site (R) button in your extension's toolbar, it may be hidden behind the "Wireframe cube" button, so click that and see if its behind there.
- This extension can access your data on some websites.