Sometimes updates breaks things, either in the software being upgraded or in some other related or unrelated software.
I have seen this many times in the past, and all kinds of automatic updates are thus disabled on my own systems, so that I can control when updates are made.
Google is wrong to force updates down people's throats. They did that with the Google Toolbar, which of course broke stuff, and for that reason I stopped using the Google Toolbar.
It's risky for the Mozilla Firefox team to implement these silent updates, but at least they give people the chance to turn it off.