You should be able to download firefox, the add-on won't stop it. But it's not compatible so it's disabled automatically in firefox.
There is no option to add an exception.
Just uncheck it in the IPS policy. "Enable Browser Intrusion Prevention for Windows". Uncheck this.
But again, the incompatibility shouldn't actually stop the download. I have ff 32 running with the browser add on installed. It just gets disabled