I've been using the Firefox docker container through the gluetun docker container (runs great with proton and mullvad) and it's been really great.
To me it's kind of like a less restricted tor browser, for when you need something stronger in terms of speed or IP blocking. And maybe something more persistent.
And it always stays open even when you close your connection.
Some of my use cases are:
Anonymously downloading larger files through the clearnet.
Anonymous ChatGPT usage.
Manually looking for torrent magnet links (though I usually do that with the tor browser)
I just use my own custom built docker images and have a few aliases set up for different "instances", e.g. one for banking, one for tis eshop, one for that eshop, etc. Each with its own firefox data dir and own downloads subfolder. Plus an alias to launch a temporary clean instance that gets discarded after it exits.
All through the same network, I'm afraid. I haven't felt the need to separate it like that, although it should be doable using docker networks, or maybe on even lower level, via Linux network namespaces.