Earlier quoted context omitted.
This is not entirely accurate. Nothing was done remotely to disable the add-ons. It happened locally. A certificate that's on your machine as part of the Firefox install expired. When that happened, add-ons that were signed via a cert chain that included the expired one started appearing to be invalidly signed . And that's why it requires an update to completely fix. That part is remote, because they need to push a n…
Clearly, downstream distributors need to create a patch which causes their distributes Firefox builds to only check certificates on add-on installation (and to check revocations too, sure): it should never be possible for a browser to fail into an unsafe configuration.
You cannot rely on check at extension install. That would assume that all malicious extensions are installed via FF proper. Oracles crapware bundling in the Java installer taught us that’s now how things go. You cannot remember the trust flag when an extension is installed via FF as a crapware installer could just set the trust flag, too. After all, that storage would be accessible, too. You cannot sign or encrypt that trust storage as the key material would have to be kept locally and would be accessible to the crapware installer.