.breadcrumb > li + li:before {
padding: 0 5px;
color: #cccccc;
content: "/\00a0";
}
Assign it the name of your icon font-family and it's UTF8 character code. As an example:
.breadcrumb > li + li:before {
padding: 0 5px;
color: #cccccc;
font-family: "icons";
content: "\e006e"; // character code for arrow right
}
Theoretically you can enter the character code into the separator variable but I haven't tested that.
You can set the font family under preferences and lookup the character code of your desired icon in the Icomoon App.