Why would you want it for Firefox, when you know it is already available in Chrome (in my opinion the better choice of the two).
Terrible answer. I suggest that you not reply if you can't give a reasonable answer.
After a new install of Avast, you should see this the first time you open Firefox:
Next you'll see this:
It's that easy.
(I'm doing this as we speak since I've just done a clean install of Avast.)
