I agree with this, except for different reasons. Without a good source of funding Mozilla won't have the resources to keep up with new requirements in the web stack. The current Firefox rendering engine isn't going to last forever and the Servo layoffs suggest Mozilla doesn't have the resources to do the R&D to keep up. Without R&D any non-monopoly product is a dead man walking.

I think Firefox's best shot for relevance is to switch from being a complete browser codebase and into being a maintained patch set for Chromium that retains user-focused features (such as adblock APIs and limits on media autoplay) as they are removed by Google. The required resource commitment would be far lower and might be viable in the long run.