Thank you for sharing additional details above.
If the firewall resource is still present, then you can try the AZ CLI command az network firewall ip-config delete command using Azure Cloud Shell. This will delete IP configuration and then you can try deleting the Firewall.
If the Firewall resource has been deleted and the Public IP is till associated to it, then in order to resolve this issue a support ticket will be required as a support engineer can request the backend team to delete the Public IP.
If you have a support plan you may file a support ticket, else could you please send an email to azcommunity@microsoft.com with the below details and I will enable a one-time free support ticket for you in this case.
Subject : Attn Chaitanya
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Please let me know once you have done the same.
Thank you!
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