By Mark Mayo, who was formerly CTO at Mozilla. Lots of great points, so I've highlighted extensively below.
A core analogy that I didn't end up including (go read it in full!) but will paraphrase.
Web engines are like the operating system market share of UNIX, Linux, and the BSDs. It's likely Chromium == Linux, and will be the basis of many important distros. Gecko is likely to be one of the old UNIX flavours, that will die off. But it might survive long term as a BSD, niche system with many small contributors. And: we need to fund next-gen approaches.
The goal and awesomeness of having an implementation of the web is to be able to nudge the whole damned web towards or away from a behaviour you care about. Make a little dent in the world, as it were. Firefox and Gecko have been able to do that, repeatedly, in some big ways and in many many many small ways, affecting the experience of every single user of the web, not just Firefox users. That’s the allure, and incredible impact, and the reason why you'd even bother with the incredibly hard, expensive, mostly thankless job of making Gecko, SpiderMonkey, and Firefox and the vast array of supporting infrastructure needed to do it. Direct mission impact. It’s important to note that Firefox has done this over multiple decades while never having “winning” marketshare. Even in its glory days it was a ~20% player. That was enough, even into the more recent single digit marketshare era, to sometimes shift the implementations of the world’s largest web platforms. Pretty rad.
 “what do we do between now and when Firefox is over?”. Because it sure feels like the end is inevitable, it’s perhaps not too far off, and that’s gonna be a weird dangerous moment for the web. I’ve thought about this going back 10+ years when I started to worry about Mozilla’s long term financial ability to maintain a full stack implementation of the web against the backdrop of slipping market share.
But for different reasons both Mozilla and Google appear far more focused on what will gain them relevance and influence in the AI wars to come, and far less on the browser wars of the past. Fair! We’re probably gonna need Mozilla.ai to be a force of nature!
the primary reason Gecko was compelling as a platform choice in the last decade wasn’t the code, IMO, it was the nature and intent of its owner. A theoretical antidote of sorts that felt good to have exist and go to battle with anyone and everyone if necessary. Mozilla Corporation, as it slowly learns to be a real company, not a movement, is a less and less obvious counter bet to the culture and behaviours of the market winners.
We desperately need organizations that deeply and primarily care about browsers and the web, who are going to do more than just skin Chromium (Linux). We need rebel alliances, and we need them to take on forks “of significance” because they are trying to influence how the web itself works for everyone, just like Mozilla did. Natural forces (fork cost) suggest these efforts are incentivized to merge over time and the web could continue to evolve and reflect multiple points of view on top of a single dominant code base this way.
Right now, and for quite some time I’d wager, Google alone decides what is de facto on the web by what they ship in Chrome, regardless of what plays out in open source code repos.
Second, someone has to fund next generation engines, in my opinion. They’re long shots, but they’re invaluable in so many ways to show us what’s possible when we don’t have the weight of every single website ever made on our shoulders. I have no idea how to fund them except all the players should just shake hands and put 1% of their fuzzily defined browser budgets into a small pool that hands out grants to crazy people.
Regardless, for those of us with the passion for building the web in some sort of way, we can’t wait for Mozilla to do anything. We’ve got to roll up our sleeves and get to work.