I rewrote the test_resource_timing test in web-platform-tests to try and fix some of the flaky behaviour, but I still see some intermittentcy in gecko. It seems that loads from an image element are sometimes given initiatorType of "other" rather than "img". Updated test is at http://w3c-test.org/resource-timing/test_resource_timing.html

I've already filed Bug 1021221 for this. It's because of caching. When the image is loaded from cache, it does not get the "img" initiatorType, so we set it to the default - "other". We have a similar issue with cached iframes, in Bug 1009360.