The W3C Navigation Timing API has found its way into most modern browsers. It is also now already party supported in Firefox mobile and fully support on new Windows Mobile Phones. While this is a great step forward it should at the moment still be used with care. We at Dynatrace started using the Navigation Timing API a while ago in our User Experience Management Solution. Just recently we discovered that that the Firefox implementation still seems to not work 100 percent correctly.
Initially we relied on feature detection to figure out whether we can use the Navigation Timing API or not. As we started to see weird results we looked at the data more closely and figured out that some of the timestamps seem to be set incorrectly. Others seems to have to have the same problems as we do. We have also seen problems with fetchStart not being properly set as well.
Obviously the information is not yet reliable enough to use it for monitoring. I am sure that this will be fixed in Firefox soon. However if you are relying on feature detection in your application to figure out whether Navigation Timing is supported you should add code to check whether the values actually make sense.