FF18.0? The current version is FF29.0! (Go to Help > About Firefox in order to update).
However your problem is probably just part of the cat-&-mouse game that Google plays with the developers of software that enables you to save Youtube videos. In order to meet their commitments to copyright holders (such as record companies and the BBC, to whom Google has to pay large sums of money to host their content on Youtube), Google has to try to prevent people from saving videos to their hard drives. (It is illegal, after all!). So they keep tweaking Youtube to prevent software such as Download Helper from working (until the developers of Download Helper catch up and tweak their own software, of course!).
So you might have to wait until there's a newer version Download Helper. Alternatively, of course, you could simply try alternative software. This is better than Download Helper anyway:
https://addons.mozilla.org/en-US/firefox/addon/easy-youtube-video-download/