first letter displays wrong in ppw5 when sync by amazone - send to kindle

[hisspop]

I have kindle ppw 5 with fw 5.16.10

I use only one code first letter css, but they display two ways before and after update firmware.

my code css:

.drop {
  display: block;
  line-height: 1.6;
  text-align: justify;
  text-indent: 0;
  margin: 0;
  padding: 0.25em 0 0;
}
.fdrop:first-letter {
  float: left;
  font-family: "font22";
  font-size: 300%;
  margin: -0.5em 0;
}

and there are the indications. How to fix ?