fireman

Fireman:
This sign uses a slight double motion. Tap the back of the hand on your forehead.

Here is another version. I recommend using the first version instead of this one.
I wouldn't "mark it wrong" on a test, but again, you are better off using the first variation.
See the discussion (bottom) for yet another variation.

Dear Student,
Not to worry, even if you do the "palm orientation" aspect of the sign "firefighter" wrong it still won't mean "b_stard" since the meaning of the sign "b_stard" depends on a complex combination of the speed of the sign, the force with which it contacts (or comes near) the forehead, the number of contacts (one vs. two) the facial expression, and the context (other signs in the sentence).
Novice ASL instructors and/or Hearing ASL instructors who learned ASL from a classroom rather than from the Deaf Community tend to make a big deal out of "near misses" between common signs and swear signs. This provides "shock" value and keeps their students interested. But in the real world, it isn't an issue.
You are better off learning the common sign for fireman instead of using lesser known variants.
Do what you need to in order to get an "A" grade from your instructor, after that, do the versions you see adult Deaf people signing at community events in your area.
Cordially,
Dr. V

p.s. By the way, thank you for sharing that variation with me.
I'll post your comment at my site. Please do be gentle with your current instructor, for all I know, that might be a very popular sign in your geographical location.

--Dr. V