All extensions disabled due to expiration of intermediate signing cert

I'm not familiar with Firefox extensions (and have pretty much stayed away from the stuff ever since they started making it "mandatory"...) but shouldn't the expiration only mean new signatures won't be valid, yet signatures made before expiration should remain so? At least that's how I understand things like Windows' driver signing works (when that was first introduced, I was quite scared that it would mean perfectly working drivers could just stop working due to the expiration, and asked... but apparently no one at Mozilla asked this question.)

Edit: wow, downvotes? Care to explain what I'm missing?

This same behavior is how certs usually work. Stuff with expired certs just does not run after the expiration date; that's because the cert tells you what server to ask for authentication, and if you have an old cert, there's no way to be sure that the original issuer is still the one in control of that domain.

You're referring to things like HTTPS and contacting a remote server, and thus your reasoning there makes sense.

I'm referring to traditional code signing, which I assume Firefox extensions are more similar to --- the goal being to ensure that some data has not changed since it was signed, and only the validity of the certificate at the time the data was signed is meaningful; even after the certificate expires, a signature created when it was valid still asserts that the data it signed has not changed.

In short, yes, they should have implemented timestamping for their code signatures like most other code signing systems do.

Without timestamping the expired cert always would have caused problems, even if it was replaced early and correctly: Every add-on would still need to be signed again with the new replacement certificate and shipped to all users. It's not as easy as just replacing the certificate on some server.

Well, this is still what has to happen: replace the certificate, ship that new certificate[1], re-sign every add-on, ship every add-on to every user.

Now, in order to ship new versions of the add-ons, you probably will have to bump the add-on version numbers as well. Which can have further unintended consequences.

[1] Incorrect, see blow; it is my understanding that the certificate in question is baked into the browser itself, with no way to push updates just for the certificate remotely other than shipping an entire new Firefox build. Well 6 new builds: esr, stable, dev, beta, nightly, unbranded. Gonna be a fun night for a lot of mozilla folks... Well, a night is not gonna be enough...

I might be wrong tho, and misunderstood something.

EDIT I was wrong (https://news.ycombinator.com/item?id=19824520), the expired cert is not baked into the browser, just into the add-on package files. No need for new Firefox binaries, after all. Still, they have to resign all add-ons and ship new versions.