The same is clearly coming for Chromium forks, which is why I've always thought the privacy and ad-blocking forks are a joke - if they ever gain enough marketshare, or if google just tires of the public open source charade, they have no chance of maintaining a modern browser on their own.
This is all the more likely now that Google has been emboldened by not having to sell off Chrome for anticompetitive reasons.
If Google does the math one day, and determines that they won't lose out anymore by not paying Firefox they'll stop paying.