Firefox also uses these collected facts and figures to render a personalized data-licious report of your browsing data such as how long you've spent on your browser, how many times it has crashed, and even how long it has taken to open (measured in milliseconds). To access this "health report," Just go to Help > Firefox Health Report.

from: http://www.pcmag.com/slideshow/story/327063/12-hidden-firefox-functions-for-browsing-like-a-boss/3