Recently, Firefox had an incident in which most add-ons stopped working. This was due to an error on our end: we let one of the certificates used to sign add-ons expire which had the effect of disabling the vast majority of add-ons. Now that we’ve fixed the problem for most users and most people’s add-ons are restored, I wanted to walk through the details of what happened, why, and how we repaired it.

https://hacks.mozilla.org/2019/05/technical-details-on-the-recent-firefox-add-on-outage/

Yes, and it happened accidently :-) Of course not.

If you look - they wanted to find way to move all people on old browser versions into new ones, and kill hundreds of old and not updated add-ons that this people used in the process. Of course the knew about certificate issue, for long years. Issue with modern versions had been arranged via not changing necessary code and not having proper certificates, as otherwise all will become too obvious and it is risky for industry.

For industry it is extremely important to keep pace of "serious threats" that they fix via constant updates. As now all large companies are symbiotically connected to hackers and provide them necessary information and financing via complex schemes (that lead to no one important).