In the case of it flipping back on, you're at the whims of bugs - just like with all software. If there's any organisation I'd trust, it's Mozilla - if only because if they make mistakes, there is a lot of pressure for them to correct this (case in point: Looking Glass).

Note that Mozilla pushes code without explicit consent for all parts of it all the time - they're called software updates. The problem in this case was that it was for a potential feature that very few people cared for, and that it showed up as a scary extension in the extension list. That definitely should not have happened, but it's not a privacy violation.