The latest Firefox 3.x actually pulls update information from an SSLized URL, and it verified the binary package before installing it. I was pretty happy when I saw that, but I started wondering how the update box below was rendered:
It turns out that the user-readable details about the new version are pulled as plaintext HTTP from Mozilla's servers. I wondered what would happen if I added a meta refresh to another download in this page. Below is the edited HTML using Charles Proxy but clearly any man-in-the-middle attacker could do the same thing.
The result of this is an arbitrary file being pushed to you at the time you are trying to update. I personally think this could be convincing enough to get people to run malicious code. An attacker could also edit the HTML directly to make it seem like you should click a button in the untrusted frame instead of the "Get the New Version" button or be even more creative.
I think it is important for software vendors like Mozilla to keep these kinds of attacks in mind when mixing application chrome and untrusted content in the same view. It is important for us to think of the barely computer literate user who is just trying to safely browse the web. I'm pretty sure more badness could be done with this, and the fix is relatively simple -- all they needed to do was serve this over SSL and the threat would be greatly reduced.