PHP - Form Post Authentication Not Working From Firefox, Suggestions Please

if(isset($_POST['submit'])) {
/* Doesn't show if i put it after if($_POST['submit'] */
if(isset($_POST['grand_total'])) {
echo $_POST['grand_total'];
}

}

//A bunch of other html/php code. Another class calculates the subtotal assigns it the variable $subtotal



$cart_totals = new cartTotals($subtotal, $discounted_amount,$post_values->tip); // Cart class is shown below

/* Doesn't show if i put it before if($_POST['submit'] */
if(isset($_POST['grand_total'])) {
echo $_POST['grand_total'];
}
echo  "<input name='grand_total' type='hidden' value='$cart_totals->grand_total'  />"; // Shows the grand total after second from submission
echo "$cart_totals->grand_total"; // Shows grand total after the first submission

class cartTotals  {

	public $subtotal;
	public $sales_tax;
        public $tip;
	public $grand_total;
        public $discount_amount;
	public $href_page;
	public $invalidCouponMessage;
	const  TEST_ENVIORMENT = FALSE;


	/**
	 * [ Function gets constructed in the order summary where the [$discount_amount= ""]  arg does need to be passed.
     * But does get passed in when called on the checkout.php page. Therefore we set the default value to an empty string.]
	 * @param [float] $subtotal   [subtotal get passed in from the parent class coreCartFunction]
	 * @param string $discount_amount [The class checkCouponCode calculates this discount amount based on the
     * subtotal and the discount amount. It gets instantiated on the clients side and passed is this construction function.
     * This is all done on the checkout page.]
	 */

    /*The way the construct function works is by invoking all the methods the passed arguments
    When the methods get invoked the do all the work and set the properties its values. The properties then get echoed out
    on the client side.  */
	
    function __construct($subtotal="", $discount_amount= "", $tip=""){
    
    $this->subTotal($subtotal, $discount_amount);//SubTotal method takes the discount amount and subtracts it from the subtotal.
    $this->salesTax($subtotal, $discount_amount);
    $this->tip = $tip;
    $this->grandTotal();
	}

    private function subTotal($subtotal,$discount_amount) {

            $rounded_subtotal = round($subtotal-$discount_amount,2);
            $money_format_subtotal =  money_format('%i',$rounded_subtotal);
            $this->subtotal = $money_format_subtotal;
    }


    private function salesTax($subtotal, $discount_amount =""){
	$sales_tax = (STORE_SALES_TAX)?(float)STORE_SALES_TAX:8.875;
	$sales_tax =(($this->subtotal)*$sales_tax)/100;
	$sales_tax = round($sales_tax,2);
	$this->sales_tax = $sales_tax;
	}


	public function Tip() {
    //global $post_values;
    //$last_tip_selected = $post_values->tip > 0 ? $post_values->tip : "" ;
	$tip_output = "<select id='tip' name='tip'>";
 	for($tip=0.00; $tip<=11.75; $tip+=0.25){
 	if( $tip == "2") {$selected = " selected";} else {$selected ="";}
       $formatted_tip =  money_format('%i',$tip);

	$tip_output .= "<option {$selected} id='selected_tip' value='$formatted_tip'>"."$".$formatted_tip ."</option>".PHP_EOL;
		}

		$tip_output .= "</select>";
		return $tip_output;

    }

    private function grandTotal(){
        $grand_total = round($this->sales_tax+$this->subtotal+$this->tip,2);
        $grand_total_formatted =  money_format('%i',$grand_total);
        $this->grand_total = $grand_total_formatted;

    }

Hello, I have this PHP and HTML code and i recently figured out, it doesn't even enter the PHP function to execute and i have no idea whats wrong

            <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" id="trashy">
                <table id="RemoveItem">
                    <tr>
                    <b><label for="Itemid">Enter the item ID you want to delete : </label></b>
                    <input type="number" name="Iid"><br><br>
                    </tr>
                    <tr>
                        <td></td>
                        <td></td>
                        <td></td>
                        <td>
                        <!-- <button type="button" class="button1" name="remove" type="submit" href="AdministratorPanel.php?click=1">Remove</button> -->
                        <input id="btn-login" class="btn btn-success" style="right: 50px;" type="submit" name="remove" value="remove"> 
                        </td>
                    </tr>
                </table>
            </form>

<?php 
  if($_SERVER["REQUEST_METHOD"] == "POST") {
      if(isset($_POST['remove'])) {
        $ido = $_GET['Iid'];
        $result = mysqli_query($conn, "DELETE FROM productinfo WHERE ID = '$ido'") or die ("error");
        if($del)
        {
            echo "success deleting record";
        }
        else
        {
            echo "Error deleting record"; // display error message if not delete
        }
      }
  }
?>