Get sessionstore JSON for a single Firefox window (including private windows)

get-sessionstore-json.js

// Run in Browser Console (Ctrl+Shift+J; ≡ menu → Web Developer → Browser Console),
// then right click on the JSON output and click Copy object.
// Use (eg.) <https://code.s.zeid.me/bin/blob/master/sessionstore2html> to convert to HTML.
SessionStore.getWindowState(window);

sessionstore2html.js

// * Change the output directory at the bottom and run in Browser Console
//   (Ctrl+Shift+J; ≡ menu → Web Developer → Browser Console).
// * <https://code.s.zeid.me/bin/blob/master/sessionstore2html> must be on your $PATH.
// * This function will replace `~/` at the start of the output directory with the
//   path to your home directory.
// * The output filename is based on the current local time and will be of the
//   format `%Y-%m-%d.%H-%M-%S.html`.
// * If the window from which you opened the Browser Console is private,
//   a `private.` prefix will be added to the output filename.

await (async function(dirname) {
 const { Subprocess } = ChromeUtils.import("resource://gre/modules/Subprocess.jsm");
 
 const notWindows = AppConstants.platform != "win";
 const pathSeparator = notWindows ? "/" : "\\";
 const home = Subprocess.getEnvironment()[notWindows ? "HOME" : "USERPROFILE"];
 if (home)
  dirname = dirname.replace(/^~[\/\\]/, home + pathSeparator);
 
 const now = new Date();
 let filename = new Date(now - now.getTimezoneOffset() * 60 * 1000).toISOString();
 filename = filename.substring(0, 19).replace(/T/g, ".").replace(/:/g, "-");
 filename += ".html";
 
 const prefix = document.documentElement.hasAttribute("privatebrowsingmode") ? "private" : "";
 filename = (prefix ? prefix + "." : "") + filename;
 
 const process = await Subprocess.call({
  command: await Subprocess.pathSearch("sessionstore2html"),
  arguments: [`--output=${dirname}/${filename}`, "-"]
 });
 await process.stdin.write(SessionStore.getWindowState(window));
 await process.stdin.close();
 return process;
})("~/path/to/tabs");