I agree, it's incredibly useful. I did a lot of research on this a few days ago, because I wanted to see if it might be possible to raise some funding to work on it. I found lots of abandoned open source projects. I'm sure there's a lot of Firefox users who would appreciate a Chromecast add-on, but there's a lot of political and economic reasons why it's probably not going to happen.

In the end I decided that it was just easier to switch back to Chrome instead of going down that road, and I'm so happy I can cast videos/tabs to my TV.