Depending on your distro, the Firefox package you see in the official repo is ... well packaged by the repo maintianers and some distros will include customizations like adding in bookmarks to the distro's welcome/docs/pages/etc. Sometimes even more things like feature settings are changed. It is usually not anything major and usually things that you might prefer like disabling pocket, etc.

What Mozilla is calling out here is a seperate, Mozilla-direct repo. This means you skip the official repos.

Everyone should think carefully about what this means.