DOM Snitch: Google's passive in-the-browser reconnaissance tool

Sites might break when you have DOM Snitch enabled. You can always right click and put it in "Standby" mode, which should solve the problem. There are at least two known issues that can cause this, listed at https://code.google.com/p/domsnitch/wiki/KnownIssues. I don't know what might be causing it to break GMail, could be a lab or that it simply was never tested much with GMail.

I wouldn't really call them rules. The tool looks for use of dangerous JavaScript APIs and, when it detects them, it gives you more information, such as stack trace and arguments. That information should help someone determine if a particular use is insecure. Besides that it has some relatively simple logic to determine if a particular usage might be a security bug or not, which is indicated through different colors.

Are you sure you enabled "Passive" or "Invasive" mode and some of the modules?

Everything will be turned off when you install it. Once you change settings all new tabs will open with the new settings.