Currently my system is saying that there are 25 mirrors in the server list that IPFire uses. The one selected to be used is chosen randomly each time it is needed. Presumably to spread the load on the mirrors. So you can’t change the mirrors that are used.
From the logs you can see how many servers IPFire had in the list to choose from when the download was carried out.
You can grep in the logs to see which mirrors were used
less /var/log/messages | grep "DOWNLOAD INFO: Host"
You can see I also had the muug.ca mirror once but also others, some of which were repeats but that can happen with a random selection out of 25 servers.
If the mirror list is more than 60 mins old then the list is recreated first before downloading anything.