And if you whack someone with a wrench until they tell you the password, it's even easier!
Seriously, if someone is getting physical access to the machine to the extent where they can remove the hard drive... I doubt that it makes a difference whether the browser's password manager keeps its passwords encrypted in-memory.
There's a huge difference in physical and criminal liability for stealing a laptop vs kidnapping someone. If tools become widespread for criminals to recover user accounts from an unpowered laptop, there's going to be an uptick in identity theft. When many cities would't even prosecute for the theft of a laptop, almost no one that's willing to steal a laptop is willing to hold someone up, which not only puts the criminal at physical risk of the victim attacking but also could result in decades of incarceration, if caught.
Seriously, if someone is getting physical access to the machine to the extent where they can remove the hard drive... I doubt that it makes a difference whether the browser's password manager keeps its passwords encrypted in-memory.