So this feature has landed in Firefox 34, but you might not be able to use it yet, because apparently the WebRTC initiator infrastructure that Mozilla uses (Loop) has been rate limited to include only a random 10% of users [1].
If you want to try Talk, and you're eager to 'help' Mozilla stress-test Loop a bit:
1. Go to about:config and set loop.throttled to false
2. Restart Firefox
3. Go to the right 'hamburger' menu, click customize and add Talk to the menu. If Talk doesn't show up, you might need to do a 'Restore defaults' first
I just tried it, and it works wonderfully, even on mobile and in an older Firefox version. The noise cancellation is not on par with Skype however.