So I use this as well, but it utterly fails the elderly grandmother test.

Its not well known. Its not easily accessibly for non-technical users. & its not clear which profile you are currently using when you are using it.

How many elderly grandmothers and other non-technical users give two shits about having multiple profiles in their browser?

I would imagine huge number of non-technical users share a computer and want their own chrome profiles so that they can access their own emails without signing out of their family members. I know my middle-aged parents use Chrome in this way for example, and it would be a blocker for switching them to Firefox.

Is this basically a use case where they don't want to create separate Windows users for some reason, but still would want their own private space in the browser?

Honestly the elderly grandmother test is meant to model discovery for all users. Not actually use meant for grandmothers or even non-technical user.

Not grandmother but my non-technical dad cares about this.

Or, just open `about:profiles` and press buttons.

They probably don't know, because Firefox doesn't bump your nose into the fact, that it has profiles.

In Chrome I can switch from one profile to another as fast as opening a new tab (i.e. instantly). Can Firefox do that?

With containers you can. It's literally opening a new tab.

If you have two profiles open at the same time like described you can easily switch desktops. The clear seperation of work and private browsing sessions helps me as well.

What is easy here? In Chrome, on macOS, it's command-` to switch windows and command-shift-m to open a new window in with a specific profile.

Also, links always open in the profile that is currently in the foreground. Is that possible in Firefox? Last I heard it isn't, but I haven't checked in a while.

Yes, there is but I don't know if it works for macOS since it works for me in Windows.

Set the profile you prefer to open for links as a default profile and make sure to tick the option to automatically use the default profile without opening the profile manager. Then for the second profile, you need to use the shortcuts for that with the argument like this

firefox.exe -P "<profile_name>"

And make sure you leave it as capitalized P, I believe that is the argument. Then apply the setting and click the shortcut. It should be opening links to the default profile that you set in the profile manager.

But it is just 2 clicks to do the same thing in Chrome (and those two clicks are really fast thanks to Fitts's law).

I use it by the same way. FF is the best!