So... with the huge caveat that I don't actually believe this... if we're going full conspiracy theory, Google probably benefits the most from Firefox being as weak as possible while staying technically alive. That is, Firefox needs to stay technically an option to stave off antitrust issues, but evil-google doesn't want anybody to actually use them, so it's preferable that Firefox is as buggy and feature-poor as possible.

Again, not something I actually believe, but the presented position is at least internally consistent/valid.