I wish Mozilla (and other browsers) also blocked plugins that don't behave while updating. I want to have java installed so I can decide to run java programs, but I don't want to have a browser plugin, so I disable it. Every single time java updates itself it enables the browser plugin again. Users should not have to put up with every installer under the sky silently installing browser plugins, or worse, updaters enabling them again.
Not sure about re-enabling, but Firefox does alert you if another program installs an addon on your behalf, and asks you to confirm. IIRC, it was about FF15 that this happened, and it also asked for ones that were installed previously.