You authorized it when you installed SP1. The extension is clearly described in the ClickOnce section of the SP1 feature list. We all read feature lists, changelogs and release notes very carefully whenever we install something this
Technically, Microsoft didn't modify a single bit of Firefox. It just dropped the extension where Firefox could find it.
Erm, this just dropped with Windows Update, automatically, with no warning or pre-authorization or feature list or anything.
Might have gone mostly unnoticed, too, if Firefox didn't pop up a dialog box on startup anytime a new plug-in is installed.