we can configure the flex-link on 3550 switches to provide the backup to a layer 2 interface.
As you have firewall connected to the ports, I would suggest use flex-link with MAC address-table move update feature if this is not configured and port 1 goes down, port 2 starts forwarding traffic. However, for a short time, switch keeps forwarding traffic to port 1 which is down.
Flex links is supported on 3550's starting with 12.2(25)SE series.So you have to upgrade the IOS on the switch to support the flex link feature.
I havenot tried it using the firewalls as you are designing, but its worth try giving a shot.This might work. use only one switch and connect 2 firewalls to it and then configure one switch uplink to firewall 1 as the primary link and the other uplink as the secondry link.
I am attaching a brief topology diagram for your refrence. Try configuring your 3550 switch with the above toplogy and configuration.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...