From the Google Blog:
Now Google’s faster than ever on Firefox and Mozilla browsers. When you do a search on these browsers, we instruct them to download your top search result in advance, so if you click on it, you’ll get to that page even more quickly.
I’m not so sure I like this idea. It’s basically the “I feel lucky” option with an extra click. On a broadband connection, would I even notice the difference? On a dial-up connection, which I had to suffer with last week, it would impose a performance penalty. I’d prefer it if this were an option.
And why only for Firefox? Is there a technical reason why this can’t be done for another browser?
Updated: The more I think about this, the less I like it. What if the top search result contains content that is objectionable? If I do a perfectly legitimate search on my work computer, I have the option to avoid downloading that page based on its summary and title. But if the page downloads for me, it goes through my company’s proxy servers, where it gets logged as something I downloaded. It’s also cached on my computer. If that page happens to include porn or other unwanted content, I could get in serious trouble and even lose my job, even though I am completely innocent.
Google Help explains how to disable this feature in Firefox:
Type “about:config” the address bar.
Scroll down to the setting “network.prefetch-next” and set the value to “False”.
The default should be off, not on, in my opinion. A browser should never, ever download content from a site that you didn’t specifically choose to visit. What are Google’s developers thinking?
Updated again: In the comments, James Grimmelmann points out:
I agree with you that this combination is dangerous and that it should probably not be on by default for users. But I think the mistake is the browser’s, not Google’s.
After reading the Mozilla Prefetching FAQ, I think James is right. I’m particularly disturbed by this part:
A web page provides a set of prefetching hints to the browser, and after the browser is finished loading the page, it begins silently prefetching specified documents and stores them in its cache… Will Mozilla prefetch documents from a different host? Yes. There is no same-origin restriction for link prefetching. Limiting prefetching to only URLs from the the same server would not offer any increased browser security.
So, if I understand this correctly, a Web page designer can stuff a whole bunch of links into a page and tag them with the “prefetch” relation type. If I click on that page, all those links will begin downloading to my computer automatically, even if they are on other servers. And if I do a Google search using Firefox, this will happen automatically for the first page in the search results list.
I really, really don’t like this. It’s especially ugly if someone identifies a browser flaw that allows code to be executed automatically from a page that triggers a buffer overflow or exploits an unpatched scripting exploit.
Yet another update: See this follow-up article.