If Mozilla is able to setup Firefox as a turn-key noob-friendly solution for a more secure and private web experience, then I can definitely see this as a sustainable business model.

For that however, many things (which are far from trivial) have to be done. Among other things:

  1. Focus on security. That means among other things: 
   1a: Reducing core code complexity (for example, removing Pocket and integrating it only as an opt-in extension)
   1b: More audits
   1c: More Rust (and Servo by that extension)
  2. Focus on integrating more privacy-enhancing features, such as:
   2a: Reducing the effectiveness of browser fingerprinting
   2b: Integrating more features from Tor-Browser into vanilla Firefox. Would be even better if they integrated Tor out of the box too. 
       In fact, Firefox should perhaps aim at 0 difference between TBB and vanilla Firefox.
   2c: Reducing reliance on (user-hostile) search engines. Perhaps integrate DuckDuckGo as default search engine in the future?
  3. Focus on marketing campaign which includes super noob-friendly (video) tutorials. I'm thinking something in the Kurzgesagt style.