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.