Oddly enough I thought Firefox had that a few years ago. I forget what I used - maybe a Mozilla extension? - but it even colour-coded tabs.
Am I wrong? Was it unofficial? A hack?
It was an official extension, distributed separately from Firefox, but it was so ubiquitous and a dependency for so many other Firefox extensions, that it would be unusual to not have it installed.
The article mentions this extension; it seems like what you're describing: https://addons.mozilla.org/en-US/firefox/addon/multi-account...
Firefox multicontainers I think it is
> For almost a decade, many of you have relied on our Multi-Account Containers extension to keep work, personal, and privacy-sensitive browsing separate without needing multiple browsers or profiles.
> We’ve heard your feedback and understood the value you find in that separation. Now, we’re bringing the power of the Multi-Account Containers extension directly into the heart of Firefox for all to benefit from.