I know Ubuntu, among others, used to ship Firefox without modules it defaulted into, so it would complain harmlessly and bring up an annoying pop-up-blocked style modal when the TTS module/engine was missing.
(they may still do this, but i'm not up to date.)
I'm on a Debian, so that ties in with what you mentioned.
It's easy to get rid of the error, I was more just curious. Thought maybe mainstream websites had started blasting speech at users as soon as they arrived.