Gigabit network with a 100 megabit firewall

In this situation, you won't need to upgrade your firewall. You're correct in that your devices will communicate with each other via the gigE switch. The presence of the 10/100 device won't have an adverse effect on your network.

Because you are using a gigabit switch (and not a hub) and otherwise all gigabit capable devices and cabling except the firewall, all your devices will communicate with each other at gigabit speeds.

Only traffic going to/from or through your firewall will be limited to 100mbps, and likely a lot lower if your internet connection is not 100mbps either, in short if your internet connection is not running at a greater speed than 100mbps then you dont need to upgrade your firewall.