quick fix:

request system snapshot media internal slice alternate

then

request system reboot

check after boot from which partition system is booted

show system storage partitions

with output of:

root> show system storage partitions
fpc0:
--------------------------------------------------------------------------
Boot Media: internal (da0)
Active Partition: da0s2a
Backup Partition: da0s1a
Currently booted from: active (da0s2a)
|
Partitions information:
Partition Size Mountpoint
s1a 183M altroot
s2a 184M /
s3d 369M /var/tmp
s3e 123M /var
s4d 62M /config
{master:0}
root>

then you can change active partition to boot from first partition

request system reboot slice alternate media internal