This is an adaption of a Lain_13 idea see here to be used in Fast Dial 1.x
The whole matter is discussed in detail in Mafi0z Tutorial Rounder Fast Dial Cells
This can also be downloaded directly into Stylish from Userstyles here
@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document url("about:blank") {
}
div.cell
{
-moz-border-radius: 25px !important;
}