[@robin-w Moderator 8 years, 11 months ago]

not tested but try adding this to your functions file

add_filter( 'bbp_get_reply_author_display_name', 'rew_display_first_name' 10, 2);

function rew_display_first_name ($author_name, $reply_id ) {
	$reply_id = bbp_get_reply_id( $reply_id );

		// User is not a guest
		if ( !bbp_is_reply_anonymous( $reply_id ) ) {

			// Get the author ID
			$author_id = bbp_get_reply_author_id( $reply_id );

			// Try to get a display name
			$author_name = get_the_author_meta( 'first_name', $author_id );

			// Fall back to user login
			if ( empty( $author_name ) ) {
				$author_name = get_the_author_meta( 'user_login', $author_id );
			}

		// User does not have an account
		} else {
			$author_name = get_post_meta( $reply_id, '_bbp_anonymous_name', true );
		}

		// If nothing could be found anywhere, use Anonymous
		if ( empty( $author_name ) )
			$author_name = __( 'Anonymous', 'bbpress' );

		// Encode possible UTF8 display names
		if ( seems_utf8( $author_name ) === false )
			$author_name = utf8_encode( $author_name );

return apply_filters( 'rew_display_first_name', $author_name, $reply_id );
}