Personally I would leave avast on the default settings, they are that way to provide a balance between protection and performance.
Given your system spec, you should have no performance issues (?)
The whole point of the resident shields is to provide protection and the script shield does more than just scan firefox browsing.
I have no idea why you would disable cloud services as that provides a valuable resource, e.g. the streaming updates for virus definition updates that shouldn't wait for the next regular update. Also the Reputation Services, helps the File System Shield, AutoSandbox and possible behavior shield.
So the real question is why would you want to do this ?