hi, you could try https://www.jeffersonscher.com/res/bookbackreader.html - despite its title it should also work for sessionrestore files.

Thanks Philipp! There I get the following error: "failed JSON parsing: SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data" and the text area stays blank. Kind regards

If you recover (undelete) a file then it isn't guaranteed that the file isn't corrupted. Especially with bigger files and possibly fragmentation chances get lower and some clusters may have been reused.

You can use this code in the Browser Console to decompress a LZ4 file.

• set devtools.chrome.enabled to true on the about:config page to enable the command line
• https://developer.mozilla.org/en/Tools/Browser_Console

var {classes:Cc,interfaces:Ci,utils:Cu} = Components;
function decompressFile(oFilePath,nFilePath){
  Cu.import("resource://gre/modules/Task.jsm");
  Cu.import("resource://gre/modules/osfile.jsm");
  return Task.spawn(function* () {
    var jsonString = yield OS.File.read(oFilePath,{compression:"lz4"});
    yield OS.File.writeAtomic(nFilePath, jsonString);})
}
// Set up file chooser
var fp = Cc["@mozilla.org/filepicker;1"].createInstance(Ci.nsIFilePicker);
var fu = Cu.import("resource://gre/modules/FileUtils.jsm").FileUtils
fp.init(window, "Open File", Ci.nsIFilePicker.modeOpen);
fp.appendFilter("Bookmarks/Session (.jsonlz4)","*.jsonlz4");
fp.appendFilter("Search Engines (.mozlz4)","*.mozlz4");
fp.appendFilter("Add-ons Files (.lz4)","*.lz4");
fp.displayDirectory = fu.File(OS.Path.join(OS.Constants.Path.profileDir, "sessionstore-backups"));
// Call file chooser
fp.open((aResult) => {
  if (aResult == Ci.nsIFilePicker.returnOK) {
  if (fp.file.exists() && fp.file.isFile() && fp.file.isReadable()) {
    var oldfile = fp.file.path;
    var newfile = oldfile + ".json";
    try {
      decompressFile(oldfile, newfile);
      console.log("Saved as: \"" + newfile + "\"");
      if(confirm("Open JSON file in a Firefox tab?")){
        var uri="file:///"+newfile.replace(/\\/g, "/");
        window.open(uri, "_blank");
      }
    }
    catch (err) {console.log(err);}
  } else {console.log("<error>");}
  } else {console.log("<canceled>");}
});