- install Cookie Auto-Delete (https://addons.mozilla.org/en-US/firefox/addon/cookie-autode...)
- set it to delete all local data for all domains, 15 seconds after its last tab is closed
- create a Firefox container for untrusted apps you can't get rid of (e.g Gmail, Facebook) and set these domains to open in this "untrusted" container by default
- set Cookie Auto-Delete not to delete the data for this particular container
- whitelist the few domains you trust so that you can keep their sessions open in the Default container
Result: No need to use a secondary browser or to install special "Google/Facebook/etc Containers for Firefox". You always browse the Web incognito by default! Only when you visit some particular webpages do you enter a custom container that keep your personal data separate from other activities.
This has worked nicely for 2+ years, with other essential extensions such as uBlock Origin, Privacy Badger, HTTPS Everywhere and Decentraleyes.