Download and decompress the xpi file, then modify the function in license.js:

async function CheckLicense()
{
  // let ticket = await browser.extPrefs.getStringValue("ticket");
  // if (!ticket) {
  //   return new BadTicket();
  // }
  // try {
  //   ticket = await VerifyTicketSignature(ticket);
  // } catch (ex) {
  //   ex.parameters = ticket;
  //   logError(ex);
  //   await browser.extPrefs.setStringValue("ticket", "");
  //   return new BadTicket();
  // }
  // ticket.end = Date.parse(ticket.end);
  // ticket.refresh = Date.parse(ticket.refresh);
  let ticket = {"licenseType":"paid","version":78,"start":"2020-08-11T00:00:00.000Z","end":"2021-08-11T00:00:00.000Z","refresh":"2021-08-11T00:00:00.000Z","email":"a@b.cc"};
  ticket.end = Date.now() + 1000*60*60*24*365;
  ticket.refresh = Date.now() + 1000*60*60*24*365;
  ticket.expiredIn = ticket.end - Date.now();
  ticket.valid = ticket.expiredIn > 0;
  ticket.requiresRefresh = ticket.valid && ticket.refresh < Date.now(); // ticket expired. poll on every call.
  if (!ticket.valid) {
    ticket.status = "expired";
  } else if (ticket.expiredIn > kSoonExpiring) {
    ticket.status = "normal";
  } else {
    ticket.status = "expiring";
  }
  return ticket;
}

And re-pack it as xpi, and install it. It will work like a charm.

Note that you may need to crack it again if thunderbird auto-upgraded it.