Firebase hosting: www.example.com redirect to example.com says "domain not verified"

I have had the same issue, example.com worked just fine but www.example.com didn't. Turned out that www.example.com needs www in the name value instead of @ for reason unknown, But @ worked for example.com.

And I didn't redirect www.example.com to example.com.

So currently I have four entries in my godaddy console for my domain, two with @ and two with www. See below. Hope that helps.

It was frustrating to get stuck in that, but after a while, I figure out Godaddy has a built in subdomain registration. To solve this problem, simply got GoDaddy->Select your domain->DNS Management->Scroll down to Forwarding and add subdomain www.

Example: