This support page [Speech Dispatcher required for SpeechSynthesis API](https://support.mozilla.org/en-US/kb/speechd-setup?as=u&utm_source=inproduct) is incomplete.
Upgrading Speech Dispatcher alone won't work. We have to make sure the currently running instance of `speech-dispatcher` is terminated and Firefox is closed or we will keep getting the same error messages.
killall -9 speech-dispatcher firefox
sudo apt-get -y reinstall --only-upgrade python3-speechd speech-dispatcher libspeechd2