Let me explain my situation at the current moment.
I almost never run Windows anymore, so up until now I didn't even know about this stunt by Microsoft in the first place. However, Firefox is still my primary browser, so while Microsoft's move is upsetting (and I want to take the time right here to say that at no point did I disagree with anything in your last paragraph), at the current moment I am far more concerned that I may have lost my control over my browser if Mozilla is able to arbitrarily disable any add-on that they want to, regardless of reason, without offering an opt-out mechanism. Firefox's update system, by contrast, is fully automatic and enabled by default yet I can still choose to disable parts or all of the update system should I want to do so. Apparently the only way to disable this is by some obscure about:config entry, which was not mentioned anywhere on the linked Mozilla page on blocked sites and that I only found out about by reading another Slashdot comment. Whether this particular add-on is supposed to be on the machine at all is, in my opinion, another concern entirely, and separate arguments can be made against it. My personal concern at the moment is Mozilla's mandatory kill switch.
At no point does any of this excuse Microsoft's silent installation of something that will harm the Web in the long run, but neither do their actions excuse the fact that Mozilla can now kill any add-on they want.