On September 30, 2021, Let’s Encrypt’s DST Root CA X3 cross-sign expired, leaving many web browsers to report that sites using Let’s Encrypt SSL certificates were “untrusted” or “unknown.”
Let’s Encrypt did, in fact, implement a new root and intermediate certificates some time ago, but after the built-in certificate stores in the Mozilla applications shipped with all versions of ArcaOS 5.0 to date (5.0 through 5.0.6) were configured. Thus, these new certificates were not included in those builds, and as a result, the new root certificate is indeed unknown.
The fix is relatively simple, and a rather common procedure for all browsers and email clients. To bring your Firefox, Thunderbird, and SeaMonkey certificate stores up to date, and learn how to do this for other new certificates as they become available, we’ve added a new wiki page, here.
Edited to add links to further information (off-site):
Let’s Encrypt – Chain of Trust
Let’s Encrypt – DST Root CA X3 Expiration (September 2021)