Yesterday, a certificate for Let's Encrypt expired, see e.g. https://docs.certifytheweb.com/docs/kb/kb-202109-letsencrypt/
That causes an error message in older versions of Firefox (44 or older) to display an error message when a site is called which uses a certificate purchased from Letsecrypt:
Your connection is not secure The owner of valid-isrgrootx1.letsencrypt.org has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website. This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate.
Updating FF is NOT an option. Opening the website with a different browser works, but in contrast to the description in the article linked above does not lead to FF loading the site correctly.
Are there any tricks to get FF working with the new Letsencrypt certificate?