I have been using a Firebox T15 on my home workstation with a branch office VPN. Do I still need to enable the Windows Defender Firewall on my workstation at home?
I would suggest still running the firewall on your home PC. The WatchGuard makes a great addition as a first line of defense, but does not completely replicate what a host based anti-virus does.
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Also, you don't say if you have implemented Gateway Anti-virus on your firewall policies.
If you haven't then the firewall will not check for virus infected files.
And, have you implemented IPS on your firewall?
If not, then again, the firewall will not check for these possible attacks, which might be detected by Windows Defender.