Chrome vs. Firefox Document Complete vs. Fully Loaded

I am doing some work comparing webpage load times between Chrome and Firefox on different devices. However, as I have done tests, I’ve noticed something weird going on with the “Document Complete” and “Fully Loaded” metrics. On Chrome, these two values are different - document complete is a lower time than fully loaded, as expected (example Result URL WebPageTest - Running web page performance and optimization tests... ). However, on Firefox, these two values are, in my experience, always the same, which does not make sense (example Result URL WebPageTest - Running web page performance and optimization tests... ). Is this a bug with Firefox, or is it supposed to work like this? I am confused as to why this is occurring.