Update: 6 September 2013. I have pushed a change to production that should work around this bug.
I got a bug report and have verified that speech is not working (except in classic mode) when using the latest release of Firefox on Windows 7. This is caused by a bug in Firefox. They know about it and have prepared a fix but it won’t be available for a while.
I’m trying to think of a work around. If you can use Google Chrome in the meantime, I recommend you do that.