Same for me on the homepage, messages.unread setter:

Firefox Aurora 24.0a2 (2013-07-19) and 2013-07-20

TypeError: templateBlock.innerText is undefined @ http://www.ractivejs.org/:217

Well, that's embarrassing. Not sure how I missed that! Fixed now, as soon as GitHub pages updates. Thanks for the heads up

Firefox 22.0 but no version of Firefox ever had the innerText property.

But yes, you're right, the problem is in the inline code, not in the library.