If the main download page automatically redirects you to the latest version (e.g., pfSense CE 2.7.x), use the following steps to find the 2.5.1 ISO: Navigate to the official pfSense download page.
Once you have the ISO, store it on a NAS or an offline hard drive. As time passes, Netgate will eventually purge all 2.5.x artifacts from their public mirrors. You are now the archivist. pfsense 251 download iso link
Here’s an interesting feature you could build or request for related to the download process for version 2.5.1 ISO: If the main download page automatically redirects you
Установка и настройка pfSense - Академия Selectel pfSense CE 2.7.x)