First I don't use the Avast Secure Browser.
I don't know where you read that, but I think it could have been the Avast Browser Extension, when you talk about it working 'alongside Firefox'.
If however, you are talking about the Avast Secure Browser (ASB), given it is this ASB sub-forum. Is a Chromium based browser and like other browsers it is possible to install more than one on your system. The issue is, if you are talking about the Avast Secure Browser (ASB), will Chrome and the Chromium based browsers work in Win7.