What's the use case for this? You can already configure different proxies to apply to different domains. The UX isn't great though, you have to use https://en.m.wikipedia.org/wiki/Proxy_auto-config

That's a really good idea. To achieve similar result, I am currently using multiple Firefox profiles which have different proxies configured. I also have separate profiles for social media. It wouldn't be bad if they made profiles work in the same way or with similar functionality like containers work (profile per tab, each tab uses different proxy etc.).

Ideally, I'd love to have Firefox handle the whole proxying or even networking part. Like have each Firefox profile use different network gateway (useful when you want to route all network traffic trough VPN for example).

I haven't tried this, but you can try: https://addons.mozilla.org/firefox/addon/sidebery/

It claims to allow you to set proxy settings per container.