I work from 2 browsers throughout the day... FF for 90% of my work related biz development, Chrome, logged-in, to Google services only.
While checking some things on Youtube this morning, I found 100% of the time, commercials in FF, while not logged into Google services, are 30 seconds long. Every time.
While logged-in to Google services (gmail etc) in Chrome, commercials, 100% of the time, were 15 seconds long.
When logged-out of Google chrome, commercials went right up to 30 seconds long again.
Interesting, not sure it useful for anything... I just found it interesting.