I am not sure I will remember that exactly ...

I started with FreeBSD with 5.4 while 5.0 was the first FreeBSD version with SMP support. I recall that I moved to 6.0 quite fast without touching the 5.5.

I have a screenshot from 2006/07 with FreeBSD 6.1-STABLE ... so I would suspect that it rather was 6.x series.