new iMac can't stream videos on safari

Remove the add-on VPN. Test again.

It wouldn’t surprise to learn some of those VPN clients have servers built to intercept and decrypt TLS-based traffic, and—even without that interception—the rest of the network traffic is a wealth of information on you that can be packaged and re-sold.

And if the TLS traffic is decrypted due to the presence of an add-on trusted root certificate, pretty much everything can be accessed if the VPN service is inclined; passwords and all. Some services can detect that interception, but many do not.

Given that the VPN credentials to most VPNs are widely known, the first-hop security is greatly diminished, too.

If you really need a first-hop protection and can’t upgrade everything to TLS, then the open-source Streisand or Algo VPN servers might interest.

What messages, errors, diagnostics, or misbehaviors are shown by Safari, when the playback fails?

Any add-on VPN apps, add-on anti-malware apps, any add-on ad-blocking tools, or similar apps installed?

Among other differences, Mozilla Firefox uses its own TLS support and its own certificate store and does not rely on the system support. Additional details would mean reversing this particular VPN install for how it hooks in, looking for differences, and looking for design or coding errors, or for shenanigans or other issues, and potentially probing the VPN server to see what it is and is not doing with its servers; whether the server is is completely benign, or logging and analyzing traffic, or is more actively intercepting and processing the data.

Apple has announced a privacy-focused VPN setup with a Tor-like design to mask IP addresses with availability planned later this year, per what was announced at WWDC 2021.