Hello Mujeeb,
it is difficult to understand what happened without more details.
You say that the second router was dropping traffic coming from internet with destination your prefixes.
Have you configured on the edge router floating static routes pointing to Null0 (with an high AD) for your prefixes in order to have it announce always your prefixes ?
I guess you have also static routes for your prefixes via the firewall.
Could the firewall os upgrade change the MAC address on the NIC ? Unlikely but possible so this could be an ARP table issue: if at least 4 hours before restoration this is enough to time out ARP entry.
Have you configured any form of unicast RPF feature on the edge routers ?
And Have the two edge routers an iBGP session between them ?
I would suggest you to provide a filtered version of your edge routers config in order to enable further investigation
Hope to help
Giuseppe