There was a push a couple years back to collect that data. Mozilla Test Pilot. They kept track of how long people kept browsers open, how many tabs they had and how they were used, opened, closed... All of this was anonymized and collected in all sorts of pretty graphs.

The site seems down, now, but that data was taken and helped shape the changes we're seeing now.

Old link: https://testpilot.mozillalabs.com/testcases/tab-open-close/a...