This extension [0] allows creating on-the-fly new containers, and delete them when you close the tab.

[0] https://addons.mozilla.org/en-US/android/addon/containers-on...

But, you wouldn't need to save a password if it was a one-off, either?

That's true, although I could picture a scenario where you want to log into a service only occasionally, so you want the password saved but don't necessarily want to keep a container for it. For instance, my wife and I have separate brokerage accounts, and I log into both simultaneously once a month.

That heavily depends on how much of a hassle the container system is, really. If specific sites automatically get opened in a specific container (a la facebook) then I'm certainly going to take advantage of password saving. Anything to have to not type and/or click fewer things to actually do what I want to do, instead of getting to what I wanted to do.

If I have to manually open the correct container first, though, that's a hassle. It's why I use containers sparingly, but hot damn do I have the "facebook container" extension installed (a site I look at maybe once a week) because automatic silos are fantastic.