export-passwords.js

// for use within chrome://passwordmgr/content/passwordManager.xul

// this is the title of the CSV data
var csv = `"hostname", "username", "password"`;

// the signons variable already exists when the document is loaded.
// looping through all entries, called e.
for (var e of signons) {
    // JSON.stringify properly adds quotes, regardless of what characters the entries.
    // so for every entry, take e.hostname, e.username, e.passwowrd and turn them into a quoted string
    // end with a newline
    csv += `${JSON.stringify(e.hostname)}, ${JSON.stringify(e.username)}, ${JSON.stringify(e.password)}\n`
}

// get a clipboard object
var clipboard = Components.classes["@mozilla.org/widget/clipboardhelper;1"]
                                   .getService(Components.interfaces.nsIClipboardHelper);
 // and copy the csv data into the clipboard
clipboard.copyString(csv);

Without comments

var csv = `"hostname", "username", "password"`;
for (var e of signons) {
    csv += `${JSON.stringify(e.hostname)}, ${JSON.stringify(e.username)}, ${JSON.stringify(e.password)}\n`
}
var clipboard = Components.classes["@mozilla.org/widget/clipboardhelper;1"]
                                   .getService(Components.interfaces.nsIClipboardHelper);
clipboard.copyString(csv);```