@amarildojr Thanks very much, useful. To confirm, that was to ensure a rollback instance based on an existing profile, yes? As opposed to a completely new instance with its own profile. I assume that namespaces could be used in the Debian not Arch case?
@deBoetie You could either create a new profile and configure it manually, or you could copy the existing "xxxx.default" contents to this newly created profile. However, you should consider configuring this new profile manually because there is no point in isolating this sandboxed profile if there are saved passwords that were copied from the default profile. I don't know much about namespaces. But the method describe in those posts were done in Arch. I assume they also work on Debian.
I was thinking in terms of keeping a pristine instance that was never used for actually browsing anywhere, but could be copied (like a template). I'll have a play with the namespaces idea in Debian when I have time.
You could add the maximum physical RAM your computer can take and use lots of VM's, one per site. With unity they would look like browser windows. I am doing this and to save hard disk space and simplify updates I have given all VM's the same non-persistent virtual hard drive so they all start identical and totally clean.
I'm actually sandboxing Iceweasel with Firejail, and set up one of them so that every change gets erased when I close the program. It saves much more memory than a VM, you should try it