There are numerous Firefox add-ons that provide this feature, e.g. https://addons.mozilla.org/en-US/firefox/addon/leechblock-ng.... IMHO, the main benefit of using Safari/Screen Time for this is that it automatically syncs the same limits across all of your iCloud-connected devices, which is great. I know Firefox has a Sync feature but I'm not sure if any of the add-ons leverage it to provide a similar seamless cross-device experience.