>And you've reported this, I assume?
This is a total non-response. They're not actually under an obligation to report a bug and use their time - Mozilla, however, is obliged to make their browser work properly, and to ensure the team that's working on it is properly staffed/resourced/competent.
Your response is downplaying someone's actual concern by acting like it's wrong that they don't spend their spare time participating in open source software development.
Furthermore, this "browser monoculture" argument is ridiculous. WebKit and Blink are both open source, and Mozilla is increasingly becoming what Opera was back with Presto: a lone engine with quirks that nobody wants to waste their time working around.
If Mozilla gave up Gecko tomorrow and forked WebKit (a la what Blink is) I don't think I'd bat an eye. This is like what junior programmers wind up learning at some point - nobody cares what the code looks like, just that it does what it's expected to.
I say this all as someone who's contributed to some Mozilla repos, has a massive personal investment in Rust (and has read a good chunk of Servo, and liked it), and has been a fan since the Mozilla Suite days.