Firefox has strong privacy protections, especially in Enhanced Tracking Protection (ETP) and Strict Mode, which can block: Third-party cookies ,Cross-site tracking cookies Storage access for embedded login frames (like Microsoft’s login.microsoftonline.com) 1. Check Firefox's Privacy Settings Go to: Settings → Privacy & Security → Enhanced Tracking Protection Set it to Standard (not Strict or Custom). This allows necessary cookies for Microsoft 365. 2. Add Microsoft Domains to Exceptions Scroll to Cookies and Site Data → Manage Exceptions and add: https://login.microsoftonline.com https://office.com https://microsoft365.com Set them to Allow. 3. Disable "Total Cookie Protection" for Microsoft Sites If you're using Firefox Containers or Strict ETP, it may isolate cookies per site. You can: Temporarily disable containers. Use a non-container tab for Microsoft 365. 4. Clear Site Data Sometimes corrupted cookies cause persistent issues: Settings → Privacy & Security → Cookies and Site Data → Clear Data   Then restart Firefox and try again. 5. Try Safe Mode Run Firefox in Troubleshoot Mode to disable extensions: Help → Troubleshoot Mode This helps rule out interference from privacy extensions like uBlock Origin, Privacy Badger, etc.