I feel like there's some implied assumption here that this change lets Google do something evil that they couldn't do before, but I don't really see what. If you're logged into Google properties (independent of browser), Google can already track you across most of the internet, via their Ads and Analytics embedded in a rather large percentage of web sites. If you don't want Google to track you on non-Google web sites…
You can have one primary firefox instance, and any number of instances that have the "--no-remote" flag set, each running different profiles. Any links you click outside of firefox (e.g. xdg-open) will use the primary firefox.
The 'firefox --no-remote -P' trick has worked for longer than chrome has existed and I've been using it to juggle about 15 profiles since firefox 3, usually having 4 different firefox instances open with 4 different profiles.
The more user-friendly way to do this is with firefox containers though.