Three Issues:
1.) Monocultures Suck: Experienced web developers know that no browser is without its deviations from W3C specifications. One of the ways that this becomes evident is when the developer observes inconsistent behavior from one browser to another. Bug reports get filed, and hopefully, just hopefully, if the browser vendor is not overrun with arrogant "WONTFIX" jerks, the behavior is corrected to conform with the standards document. In a monoculture, this doesn't happen as often, and gradually, the sole-surviving implementation displaces the documented standard, creating a significant barrier to the creation of alternative implementations in the event that people start to crave competition again. Instead of implementing the standard, an alternative browser now has to reverse engineer and mimic all of the bugs in the dominant rendering engine, so as to be compatible with the same web content.
2.) Mozilla happens to be a "Protector of the Web", and the "Narrative" is Appropriate: One of the great virtues of Mozilla is that, in addition to being a non-proffit organization, they aren't an operator of any major web properties. As such, they aren't subject to the conflicts of interest that you often see with companies like Google and Microsoft, who are often tempted to tailor their browsers to their commercial interests: interests that may be at odds those of the user.
3.) As of early 2019, Firefox Significantly Outperforms Chromium: Has Auchenberg even tried Firefox in the past year? Ever since the release of Firefox Quantum, Firefox has been blowing the pants off Chrome. Better yet, its Servo rendering engine is written in Rust, a modern language with safety guarantees that aren't achievable in C++. Mozila's leadership with Rust points to the possibility that we will one day be able to have some confidence in the security of our computing environments. Sticking with C++ is not the path forward if we hope to ever fully trust complex software like browsers.