TLS certificate durations turn out to be complicated and subtle
The surprising TLS news of the time interval is that Let's Encrypt made a systematic mistake in issuing all of their TLS certificates for years. Instead of being valid for exactly 90 days, Let's Encrypt certificates were valid for 90 days plus one second. This isn't a violation of the general requirements for Certificate Authorities on how long TLS certificates can be, but it was a violation of Let's Encrypt's own certificate policy.
TLS certificates have a 'not before' and 'not after' times. For ages, Let's Encrypt (and almost everyone else) has been generating these times by taking a start time and adding whatever duration to it. You can see an example of this in some completely unrelated code in my entry on how TLS certificates have two internal representations of time, where the certificate starts and ends on the same hour, minute, and second (19:40:26 in the entry). However, it turns out that the TLS certificate time range includes both the start and the end times; it's not 'from the start time up to but not including the end time'. Since this includes both the second at the start and the second at the end, a simple 'start time plus duration' is one second too long.
(A properly issued literal 90 day certificate from Let's Encrypt now has an ending seconds value that's one second lower than it starts, for example having a not before of 2021-06-10 15:31:37 UTC and a not after of 2021-09-08 15:31:36.)
This is already a tricky issue but the Mozilla bug gets into an even more tricky one, which is fractional seconds. If a certificate has a 'not after' of 15:31:36, is it valid right up until 15:31:37.000, or does it stop being valid at some time after 15:31:36.000 but before 15:31:37.000? The current answer is that it's valid all the way up to but not including 15:31:37.000, per Ryan Sleevi's comment, but there's some discussion of that view in general and it's possible there will be a revision to consider these times to be instants.
(People are by and large ignoring leap seconds, because everyone ignores them.)
All of this careful definition of not before and not after is in the abstract of RFCs and requirements for Certificate Authorities, but not necessarily in what actual software does. Some versions of OpenSSL apparently treat both the not before and not after times as exclusive when validating TLS certificates (cf); the time must be after the not before time and before the not after time. Other software may have similar issues, especially treating the not after time as the point where the certificate becomes invalid. I would like to say that it also doesn't matter in actual practice, but with TLS's luck someone is eventually going to find an attack that exploits this. Weird things happen in the TLS world.
PS: Let's Encrypt's just updated CPS deals with the whole issue by simply saying they will issue certificates for less than 100 days.
PPS: Some certificate reporting software may not even print the seconds for the not before and not after fields. I can't entirely blame it, even though that's currently a bit inconvenient.