I added a bunch of plugins to Firefox before trying bitwarden. Got the error above.
Uninstalled the previous version, deleted the appdata local and roaming folders and registry entries. Installed the latest Firefox. Firefox 85.0.1 (64-bit), Windows 10 Pro 1909. In network settings I set Enable DNS over HTTPS and NextDNS as the provider. So no Cloudflare.
MS Edge logs in fine.
Any idea(s) where I should look next?