How can I tell what version of FreeBSD my pfSense version is based on
How can I tell which version of FreeBSD was used as a base for a specific version of pfSense?
The Versions of pfSense and FreeBSD article contains a table of the versions used in various releases, or follow these instructions:
From the Dashboard in the GUI, look at the System Information widget. It is displayed under the Version section. Clicking the FreeBSD version displays the full kernel uname -a output.