It's common in many English dialects to read /θ/ as /f/. So it's possible for people to pronounce like that too. Of course I know almost nothing about your dialect, so that's only a guess. Check http://en.wikipedia.org/wiki/Th-fronting for more info on that.
In terms of linguistics, [s], [θ] and [f] in Min Dong are three allophones that belong to the same phoneme. In other words, these three allophones never serve to distinguish different characters or meanings. That's why there's only ONE letter "S" representing this phoneme in Foochow Romanized.
Such phenomenon also exists in Min Nan. The interchangeable [dz]/[l] is a good example.
Three C's define me: Chinese by birth; Canadian by choice; Christian by grace.