You can capture application usage in Casper, assuming its enabled in your JSS. It will actually show usage pie charts per machine in the JSS of open applications (all apps, not just browsers), but unfortunately exporting reports on it to something else that can be viewed outside of the JSS UI directly isn't really possible. You can take screengrabs, but since the date is only viewable per Mac, it would be highly impractical.
Even it it could be exported, I've never seen this feature be very reliable. I have seen many Macs fully checking in for months on end with very little data to report on usage, while other similar machines will have multiple days of pie charts. I'm not sure why it will work fine on some Macs and not on others.
The only other thing I can suggest at the moment would be to build some Extension Attribute scripts that can pull application launch dates using mdls (Spotlight) per browser. Its possible to build an EA that will show app use/launches in a given period of days, meaning number of launches per past 7 days, past 14 days, etc. I've done some EAs like that.
The main issue with doing the above with a browser is that its highly possible for clients to leave browsers running for days on end without quitting them, so it would throw off an EA like the above, since mdls will only capture actual launches of the application, not if its been left open over the course of 24 hours.
