Earlier quoted context omitted.
Can you expand on how you set up the profile + containers + vpn and how that works for you?
Think he means this addon: https://addons.mozilla.org/en-US/firefox/addon/multi-account... At least that's for controlling them easily. I use it for Whatsapp web on 2 accounts
For proxies you can use whatever vpn service or plugin[1]. I use paid Nord because I need consistent access to certain countries for testing. There is some tunnel bear/Nord like plugins that have lists of free proxies.
1. https://addons.mozilla.org/en-US/firefox/addon/proxy-switche...