Well to be fair, if google wanted to kill Firefox they could just stop paying Mozilla for using their search engine as the default. That's basically the only thing that's keeping them afloat.
They don't want to "kill Firefox" though. They want people to use Google products in all forms - using Google through Firefox is better than scattering users to other browsers without a Google default. By forcing users to Chromium they don't just kill Firefox, they direct users TO ever more Google products in the process.