It's always felt somewhat unsettling that, although the Internet is decentralised, a small number of large organisations have a surprisingly large amount of control over the software which the majority of users view sites with. I don't like this concentration of power --- even if you think Mozilla is benevolent, it's not immune to making mistakes; and the more power it has, the bigger the consequences.
Before the forced code signing, before the automatic updates, Mozilla or any other organisation's mistakes would not have such dramatic effects; now, they have the power to basically break almost all their userbase nearly instantly, and that is what worries me the most.