I am happy to announce that the latest release of Qubes Mirage Firewall has just been released (0.9.4). That release fixes an issue where where the firewall was unable to communicate with its netvm when the netvm was not responding to ARP requests. This was seen by @qEawma5f with a Mullvad VPN AppVM, but could occur in other contexts. Now, in the case where the mac address cannot be resolved by ARP, we use the same permanent mac address fe:ff:ff:ff:ff:ff that is currently already added to the linux guest cache. So if you have experienced similar issues in the past, maybe this is working for you now 
You can update either manually (local compilation with podman/docker, or download from github, check hashsum, copy to dom0) or with the salt formula available on the github repository, or with @unman’s tool.
If you have any comments, please let us know, on this forum or on github.