Earlier quoted context omitted.
The major problem is that they installed an add-on without properly communicating what it was. A somewhat smaller problem but still a big problem is that was an utterly frivolous add-on that shouldn't have been pushed to people who didn't explicitly want it. But the biggest problem is that Mozilla seems to have trouble understanding why any of those two would be a problem, I want my browser vendor to be serious and n…
The major problem was building a feature into the product that allowed for pushing add-ons without users knowledge much less active consent in the first place, there is no benign use for this kind of functionality.
Security updates were and still are configurable to be installed after prompting, also when they are installed automatically I am notified that this has happened. There is also an implicit trust in the vendor that only security-related functionality should be changed in a security update.