The simplest solution for cookies isolation is to use different user profiles
to access the websites that you wish to isolate, with differing desktop shortcuts
For Firefox, you have already found the Multifox extension that allows
connection to websites using different user names simultaneously, although manually.
For Chrome, a better site isolation mechanism is built-in,
currently in various stages of development or experimentation.
See these articles :
The Chromium Projects - Site Isolation
The Chromium Projects - Isolated Sites
30 days with isolated apps in Chrome
I have personally never tried Site Isolation, but as all Google websites are in the google.com domain, I have some doubts.
From the article Sibling domains cookies isolation :
Sibling domains are subdomains that share a common suffix which is not
a public suffix. Mozilla maintains a list of public suffixes that
is used by Firefox, Chrome and Opera. IE has its own list.
Browsers implement weaker isolation policy for sibling domains than
for completely separate domains: a site can set cookies for its
Chrome's Site Isolation seems like your best bet and my last link contains an example
of how to set it up, but I do not know whether this will override sibling domains.