This is not specifically Qubes related, but others here may also be using Whonix’s SecBrowser.
Tor Browser 10 has been released and SecBrowser can be updated to it via download-secbrowser. However, thereafter SecBrowser will start seemingly normally but not connect to any site.
Until a fix comes from Whonix and/or Tor Browser, the workaround is to set preference “network.dns.disabled” to false.