There is still a possibility that Mozilla's improvements can break backwards compatibility. One time they attempted to improve Firefox's error message for undefined property accesses in JavaScript, but it caused breaking changes because some websites relied on parsing the old error message format.

Mozilla's response was to roll back the change instead of blaming the users. The alternative was letting Firefox lose market share to Chrome because the websites people wanted to use didn't work. At least one major e-commerce website (Flipkart) was actively losing sales because of the bug.

Mozilla trying to strongarm people into using web features the "right" way sounds like something Google would do.

https://bugzilla.mozilla.org/show_bug.cgi?id=1488417

https://news.ycombinator.com/item?id=19492660