The Linux desktop Firefox sandbox is pretty strong AFAICT. All the usual namespace protection, pretty good seccomp filter. I wonder what the issue they're referring to is and whether that comment is up to date (the sandbox has improved a lot over the last few years).

The WebView argument is basically an argument for monoculture: you should only have one browser engine installed because that's less attack surface than two, and since Chromium must be installed for reasons, it must be Chromium. That's actually kind of a weak argument since it's not that common for users to load a specific attack in both browser engines.