What avast version are you using (9.0.2013 being the latest) ?
Do you have any specific youtube examples, I have just updated firefox to 27.0, I can't check Facebook as I don't have an account.
How do you know it is scanning less that would I imagine be difficult to determine what was previously scanned against what is now scanned.
EDIT: During the course of the day prior to update of firefox my statistics state 70,000+ scanned items for the web shield and 800 by the file system shield. I stopped and restarted the web shield to zero the web shield count and monitor it.