(I implement this second stock Firefox environment not with a profile but by changing the
$HOME environment variable before I run this Firefox. Firefox on Unix helpfully respects
$HOME and having the two environments be completely separate avoids various potential problems.)
In theory perhaps I could use something like Firefox Multi-Account Containers for this, but I prefer separate profiles. They’re probably more work to set up in the beginning, but I feel more confident about what they’re doing and how various aspects interact with each other. Profiles are definitely completely separate from each other, including for extension settings and so on. Containers, well, it depends.
Sidebar: my usual Firefox addons for these profiles
In addition to uBlock Origin and Cookie AutoDelete, I also add Foxy Gestures (which is a critical part of my Firefox experience), Cookie Quick Manager, and generally Certainly Something (for TLS certificate viewing). Some of these profiles have accumulated additional addons over time, usually ones that mitigate tracking. These days I also install my bookmarklet buttons (for readable and fixed).