It will ask you which browser you wish to have as a default. You can easily switch from one to the other if that is what you wish to do.

You don't have to turn anything off or remove anything from your computer. If you install Firefox, you can use it as a second browser if you want. For myself, I have gutted IE off my system. I don't recommend anyone do that unless you know what you are doing. Firefox is now my main browser.

I can also view Daz readme's just fine in Firefox too.

You don't need or shouldn't need IE for that.