Of course, other dialects than just that of the Quakers have retained the familiar forms; here in Yorkshire they are still widely used, though not consistently; it's common to mix familiar and polite forms within a sentence.  Around Sheffield, "thou" has become "tha" ["tha knows" = "thou knowest"] and "thy", "thi" ["thi'sen" = "thyself"].  "Thee" remains the same, generally, though in some constructions it gets shortened almost to "thi".

Malcolm