Short answer
Make sign-in same-site with your app. If you host on Firebase Hosting with a custom domain, set authDomain to that domain. Otherwise proxy the auth helper paths to your firebaseapp.com domain or self-host the helper code, as Firebase’s redirect best-practices guide describes. If sign-in still fails in a current regular browser window, the browser itself is the cause.
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Why the sign-in iframe can’t use storage, and the fix
01
The auth iframe can’t reach browser storage
“This browser is not supported or 3rd party cookies and data may be disabled.” Firebase, JS SDK auth errors (web-storage-unsupported)
The popup flow checks through an iframe on your authDomain whether web storage works, and rejects with this error if it doesn’t. Firebase’s message names two possibilities: the browser is not supported, or third-party cookies and data are disabled. If your app runs on your own domain while authDomain is still the default firebaseapp.com domain, the iframe is cross-site, and Firebase says that mechanism doesn’t work with browsers that block third-party storage access.
02
The app is on a custom domain but authDomain was never changed
“Update your Firebase config to use your custom domain as your authDomain” Firebase, Redirect best practices (Option 1)
On Firebase Hosting with a custom domain, set authDomain in the Firebase config to that custom domain so the helper runs on your own site.
03
The app is hosted outside Firebase
“Another way to eliminate the cross-origin storage access is to self-host” Firebase, Redirect best practices (Option 4)
On Vercel, Netlify or your own server, proxy the auth requests to your firebaseapp.com domain or self-host the sign-in helper code, then point authDomain at your domain.
04
The browser can’t use storage at all
The error message also allows that the browser is simply unsupported. Try the same sign-in in a current Chrome, Safari or Firefox outside private mode and outside an in-app browser. If it works there, the environment is the cause, and changing authDomain will not help.
05
Asking users to change browser settings
“asking your users to disable storage partitioning features on the browser is rarely an option” Firebase, Redirect best practices
Also check: telling users to turn off storage protections does not scale. Firebase says asking users to disable storage partitioning is rarely an option.
Checked against the vendor’s docs on : Firebase docs: Best practices for signInWithRedirect on browsers that block third-party storage; Firebase JS SDK source: auth errors; Firebase JS SDK source: popup sign-in.
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Related: Firebase auth/popup-closed-by-user · Firebase Google sign-in popup blocked · Firebase auth/unauthorized-domain · Firebase auth/unauthorized-continue-uri