I think this is just using SpeechDispatcher from the system -- so it's not a Firefox specific thing. I get a similar (but very slightly different) voice on my own system by default -- which matches what I get when I run a command like spd-say --wait "Hello world" from the command line.

I'm pretty sure SpeechDispatcher can be configured to use a different synthesis engine -- Arch's wiki has some suggestions: https://wiki.archlinux.org/title/Speech_dispatcher -- but I haven't dug into it yet.

Yeah, in my opinion this sounds worse than DECtalk from all the way back in 1983 (for instance, Perfect Paul). Or MacinTalk (see its usage in Whatever Happened to Robot Jones, particularly the original version).

The newer stuff often tries to sound less robotic but then if it doesn't have the complexity to sound realistic it just sounds worse, harder to understand and yet also sounding uglier... warbly.

The best part is when it fucks up the HDMI audio output of your expensive graphics card.

Yes, I've experienced the same issue as the guy who posted the solution.