Code Is Deleting Header

[zero635]

I have the following code that will delete every row that doesn't meet the specified criteria. However, when I run it, it deletes the header which is in row A1 thru Y1. Can you tell me what I need to put in this code to stop it before it gets to the header?

Sub DeleteRows()
Application.ScreenUpdating = False

Dim ws1 As Worksheet
Dim Firstrow As Long
Dim Lastrow As Long
Dim Lrow As Long

Set ws1 = ActiveSheet

With ws1

    Firstrow = .UsedRange.Cells(2).Row
    Lastrow = .UsedRange.Rows(.UsedRange.Rows.Count).Row

    For Lrow = Lastrow To Firstrow Step -1
       With .Cells(Lrow, "x")
               If .Value <> "4SK5" And .Value <> "4S52" Then .EntireRow.Delete
       End With

    Next Lrow

End With

Application.ScreenUpdating = True

End Sub

Any help would be appreciated.