You can import Bookmarks from an HTML File. If you export your bookmarks to HTML you can see the structure to work with, but it can be pared down quite a bit:

 <!DOCTYPE NETSCAPE-Bookmark-file-1>
 <H1>Bookmarks Menu</H1>
 <DL>
 <DT><A HREF="http://www.yahoo.com">Link in main Bookmarks menu</A>
 </DL>
 <DT><H3>SubFolder 1</H3>
 <DT><H3>SubFolder 2</H3>
 <DT><H3>SubFolder 3</H3>
 <DL>
 <DT><A HREF="http://www.yahoo.com">Link in Subfolder 3</A>
 </DL>
 <DT><H3>SubFolder 4</H3>

additional information:

Firefox stores bookmarks in a single Sqlite file - places.sqlite; and HTML is a "transactional" file type in Firefox (used for importing and exporting bookmarks in a "common" format).

http://www.sqlite.org/

Thank you! If I make it in a spreadsheet and export it to html, do I just add that coding to the front end of it? and how do I specify sub-sub folders? and sub-sub-sub folders? i.e. folders within folders within folders?

I'm not sure exactly how the HTML would come out of a spreadsheet application, so your mileage may vary there.

The forum has sadly stripped the indentation out, which would make it easier to read how folders relate, so I've placed a further example with indentation and sub-sub-folders on PasteBin. Hopefully this clarifies how to create your own: http://pastebin.com/Gv8BBSgD

A possibility is to use JavaScript in Firefox to create the main structure and edit that file.

Run the script as a bookmark or in the Scratchpad with an about:blank page in the current tab.

var head = '<!DOCTYPE NETSCAPE-Bookmark-file-1>';
var meta = '<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8">';
var title = '<TITLE>Bookmarks</TITLE>';
var menu = '<H1>Bookmarks Menu</H1>';

var dls = '<DL>';
var dle = '</DL>';
var folder = '<DT><H3>Folder #</H3>';
var link = '<DT><A HREF="http://">Link #1 in Folder #2</A>';
var text = [];

var maxF = 100;
var maxL = 1;

text.push(head);
text.push(meta);
text.push(title);
text.push("");

text.push(menu);
text.push(dls);
text.push("");

for(var i=0; i<maxF; i++){
text.push(folder.replace(/#/, i+1));
text.push(dls);

for(var j=0; j<maxL; j++){ 
text.push(link.replace(/#1/, j+1).replace(/#2/, i+1));
}

text.push(dle);
text.push("");
}

text.push(dle);
text=text.join("\n");

var URI="data:text/html;charset=utf-8,";
location.href = URI + text;