Amazon divides its cloud into multiple sections. This page is explaining the connections between those and tries to give advise on where to deploy your infrastructure.
The largest one is called region and is a cluster of data centers. One region is us-east-1 which is located in North Virginia in the United States. Another one is eu-west-1 which is located in Ireland.
There are multiple factors that influence choosing the right region.
Most importantly is proximity to where you and people who are using your hosted services are. Then, there is cost which varies in all the regions. There might also be legal considerations on from where you can offer your services.
Technically, there is no difference between the different regions and IPFire is available in all of them.
Each region has multiple availability zones. An availability zone (AZ) is a logical group of data centers within a region. All AZs are entirely independent of each other - meaning that if one AZ is unavailable, all others should not experience the same problems. For applications that need to be highly available, it makes sense to have them running redundantly in multiple AZs.
In all regions, all availability zones are equal. There is no reason to prefer one over another one. For installing IPFire you will need to pick one and create all subnets and the IPFire instance in it.
If you need to host your services in a second AZ in case the first one goes down, you should stay in the same region if you can. IPFire can be used to connect multiple AZs via VPN.
There is usually no need to engage a third availability zone as availability of Amazon's data centers is quite good and its design makes sure that there is usually not an entire region failing.