I won't strongly argue with your opinion. Basically because business is complicated and I don't believe I would know what is the losing strategy for Mozilla. But I don't really know how Mozilla could be interesting to me outside of Firefox.
Someday they had a name which could be worth something, but I lost my trust in Mozilla ages ago. I don't believe their management, to me they have as much integrity as Facebook (ok, maybe a little more... I'm not sure). I am using Thunderbird and don't know a good alternative, so it would be a pity if it dies, but I'll get over it pretty quickly. Moreover, I don't really need any new features for Thunderbird, nothing but security updates.
Even more than that, I am not really that picky about the engine. Sure, I was always pretty enthusiastic about the technology in Mozilla (I mean — Rust! — do I even need to say more?). And I do like Gecko. But, you know, if they believe they will deliver better if they switch to WebKit, I'm kinda ok with it. If it won't break any plugins, chances are I won't even notice. I may cheer for Servo and stuff, but it's not why I use Firefox. The browser as a product is a bigger part of the UX for me than mere rendering engine. Also, Firefox is still a little less of a spyware than the competition. Fuck up that experience for me, and you lose a very loyal old-time user. Keep up with the competition while not fucking it up, and there's still enough momentum for me not to switch. I guess there's a lot of people like me in that regard.
Finally, Mozilla is not a startup with a core of energetic talented people, who can make miracles happen. It is a quite old non-profit organisation, entrenched with politics. Their biggest (only?) asset I know of is people, who know browser stuff. Well, maybe the brand, known for browser stuff. Do they have anything else at all?
So, yeah, anything is a bet, but I don't think I would bet on anything outside of Firefox. Not now, anyway. When everything is good financially and we have money to spare, well, looking for alternatives may be good. But when you are struggling for money and are about to lose everything you were keeping afloat for the last 20 years... As I said, Mozilla is not known for brilliant top-management, that has a crystal-clear vision and makes ingenious decisions that save the company when the business is struggling. The best thing I would expect them to be able to do is cutting costs, finding new sponsors and new ways to "slightly sell" Firefox without angering too many users. And even that they don't do very well.
The most radical thing I would reasonably expect them to pull of would be to switch all forces to mobile version of Firefox and win it, while loosing desktop version. But I doubt it.
[Also, in some parallel reality they could take advantage of political situation and sell Firefox OS to Huawei (or maybe get a sponsorship). But in this reality, Firefox OS is dead, Mozilla is too rigid to make wild moves like that, and Huawei is waaay more capable of building their own OS than Mozilla.]