Making a breaking change to the mobile API also breaks old outdated installations of the app. Websites and their APIs are usually synced, apps not so.
If they were really motivated to stop your method, they could just obfuscate the frontend with webpack and break your scraper every time they make an update.
Mozilla’s next largest source of revenue is subscriptions and advertising (source 2021 financial report), by a wide margin. That “useless shit” is their other revenue, and they’re investing in it because they know they need to diversify revenue to fund Firefox. You’re suggesting they kill it because it’s not their core (unprofitable) business?