The linked article was posted on January 6th, 2020. For some reason many people received an email about it today. Must have just been a soft rollout and now they are starting to actually publicize the feature?

Yeah, I've been able to use it for months (FF on Mac)... Was it rolled out to certain computers for testing before?

Same here. FF on Mac. I’ve been using it since lockdown began in the UK in March. It’s been really handy to keep BBC News in the corner of the screen when the Government was doing daily briefings.

Firefox on Ubuntu here, also for months at least - don't remember when exactly though.