I believe that a viable strategy for them would be to instead focus on making Firefox better than Chrome on everything else but the Google Stack. If the Firefox experience is better on all other sites, Chrome will be gradually marginalized. It will be just a program that you keep open to do "work", similar to running MS-Office, while Firefox which will be considered the main web browser. This approach would keep the Google cash (from the default search engine deal) flowing.
But they should also work on alternative sources of income. A Firefox Account that syncs passwords (Lockwise), bookmarks (Sync) and provides file sharing capabilities (Firefox Send) is a service worth paying for. But they have to put some work on properly integrating all these features. And most importantly, they need to get rid from the lock-in mentality and support all the services tied to the Firefox Account on competing browsers by developing/porting the corresponding extensions.