Both function pointers and "first class functions" refer to function indirection.
In its treatment of expressions, C doesn't draw the distinction between function and pointer to function. When you call printf("foo\n"), the printf part is a primary expression which designates a function, and evaluates to a function pointer. That pointer is then dereferenced by the postfix ().
The main difference between a "first class" function and a function pointer is that a first class function carries an environment, which allows the body of the function to make references to lexically scoped names outside of the function. A function pointer carries a reference to the code only.
In its treatment of expressions, C doesn't draw the distinction between function and pointer to function. When you call printf("foo\n"), the printf part is a primary expression which designates a function, and evaluates to a function pointer. That pointer is then dereferenced by the postfix ().
The main difference between a "first class" function and a function pointer is that a first class function carries an environment, which allows the body of the function to make references to lexically scoped names outside of the function. A function pointer carries a reference to the code only.