Chrome has this feature with dedicated profiles, and you can accomplish this in firefox pretty easily using some good extensions.

Yeah, but that's really a hassle and such a workaround shouldn't be required.

And firefox's extensibility is great as usual, but its bloat and the new UI and the ads on the tab pages disillusioned me

In chrome, you can make a shortcut for a profile by providing a flag --profile-directory="YourProfileName" It will be created if it doesn't exist and you can even set it to point to a TEMP folder for throwaways.

Firefox can do this with separate profiles too, so long as you also include --no-remote with other instances.