Comment by garyrob
After upgrading to Sequoia, I could not browse with Safari or Mozilla. What fixed it for me was to go to the DNS settings for my Wi-Fi connection, and add Google's DNS servers (8.8.8.8. and 8.8.4.4). They replaced the autofilled DNS servers that were there.
Were the autofilled DNS servers in RFC1918 private space (10.0.0.0/8, 192.168.0.0/16, etc.)? I had issues after the upgrade with Google Chrome being unable to access hosts in these ranges, and fixed it by going to System Settings -> Privacy & Security -> Local Network and toggling Google Chrome off and on again.