Mozilla uses a system addon called Web Compatibility that allows them to create and distribute mini-patches for popular sites. These fixes can be as simple as changing the browser’s user agent when visiting that site or injecting custom CSS and JavaScript into the page. As of Firefox 68, users can now view all of the fixes that were created for sites under the WebCompat system. This makes it easier to update our webcompat interventions as sites change, without needing to bake those fixes directly into Firefox itself.
Source: https://www.bleepingcomputer.com/news/software/firefox-about-compat-page-shows-fixes-for-reported-site-issues/