[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.

[greyface-]

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.

[garyrob]

No, they weren't local. I have no idea where they came from. I couldn't even delete them, but when I added the Google servers, they autofilled ones were automatically deleted.

[kccqzy]

They came from DHCP or RDNSS in RA.

[JumpCrisscross]

> could not browse with Safari or Mozilla

FYI, it looks like Firefox fixed this.

[zakki]

So not macOS error?

[JumpCrisscross]

> not macOS error?

It worked before I upgraded to Sequoia. But I don't know enough to point fingers. Just mentioning that turning off the firewall long enough for Firefox to update fixes the problem.