Can you elaborate how you sandboxed tabs in Firefox? I use incognito when using Google or sites that use Facebook commenting on order to lower the tracking ability, but sandboxing sounds better.
Then get Cookie AutoDelete, tick "Enable Support for Firefox's Container Tabs", and whitelist the cookies you need to sign into those sites.
Combined with uBlock Origin it's the best setup I've found so far, though Firefox Quantum destroys my MacBook's battery.
I used to have Safari and Safari Technology Preview to keep good battery life, with only one browser signed into Google, FB and others but I still noticed some searches into the non-signed in browser leaking to FB - I have a feeling the two browsers share some state.
If anybody knows an improvement for Firefox's battery usage on macOS I'd love to hear!