You can use pseudo-elements to include text after an element and you can use CSS3 selectors to remove the trailing slash from the last element.
#footer_menu li:after {
content: "https://stackoverflow.com/";
}
#footer_menu li:last-child:after {
content: "";
}
EDIT:
The whole thing can be done in one line with better CSS3.
#footer_menu li:nth-child(n+2):before {
content: "https://stackoverflow.com/";
}
EDIT: EDIT:
It’s even easier than that.
#footer_menu li + li:before {
content: "https://stackoverflow.com/";
}