Firefox (and its forks) have an integrated profile manager, though it’s not always intuitive to figure out how to get to it. LibreWolf is the fork I seem to always go back to, and it has zero slop.

I use containers. Right-click on the new tab button and pick a container to open the tab in. There’s also an add-on that will do this automatically for you when you visit a specific website, so if you want every site to live in its own container, you can do that too.

Personally I just use its built-in cross-site cookie blocking, but multiple ways to do the same thing.