Do you mean you want it to look like this:
If so, you have to replace the Google search provider icon by editing the google.xml using base64, like you previously did, but this time use this code to replace it with a clear icon (yeah, I know, it's really long):
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And then after you restart Firefox, put this code into Stylish to remove the drop down marker:
/* Hide the Search bar dropmarker */
.searchbar-dropmarker-image {
display: none !important;
}