It would make more sense to turn it ON, but Google is doing it differently. Prior to any major release they are running Chrome engine through a lot of locally cached sites, from most popular down, approx. 1 million of them. Then software reports back which sites failed the test and programmers can focus on that, see what went wrong and how to optimize engine further. It is much faster process than public beta testing, results are more accurate and many times it also helps them improve coding practices, not only fixing particular bugs.
If you are really paranoid or want to be 100% sure, then you can use packet sniffer like Wireshark [ http://www.wireshark.org/ ] and again, see for yourself. Packet sniffers support filters and trigger alarm only when certain IP, domain or certain text is present inside the packet. You can also limit sniffing to one application only and keep logs for easier searching. I am many times surprised at people how they will believe just everyting, even programmers, like one colleague of mine said Microsoft can see everthing you have on PC, which is nonsense and not true.
You need to be aware of the simple fact, each popular product is tested for that MANY TIMES, not only by users, but also by competitors. It is easy way to destroy reputation of someone if you discover security flaws/leaks inside their products, and you can be sure Mozilla and others want to see that. After all they are threatened by a superior product, and on top of that it has no commie imagery*, which is apparently ideological threat to them as well.
*Commie stuff @ Mozilla/Firefox:
http://freerepublic.com/focus/f-news/1149061/posts
http://www-archive.mozilla.org/party/2002/flyer.html
http://www.mozillazine.org/talkback.html?article=3164#9
http://www.courageunfettered.com/stuf/mozillapics/
http://www.courageunfettered.com/stuf/mozillazine/
