The company says there are still 12 million users of 3.5, which debuted in early 2009.
Firefox 3.5 is no longer supported, and the last security patch was sent out last month.
Says Christian Legnitto, Firefox release manager:
We are treating the automatic update checkbox enabled as a 'Yes, I want Mozilla to keep me updated. Previously as a courtesy we had people opt-in between major versions due to the potential jarring nature of the update. We feel the difference between 3.5 and 3.6 is not severe and with 3.5 reaching end-of-life 3.6 is the security update for 3.5 users.
The browser company says most users have auto-updates selected.
Mozilla does admit they would prefer that 3.6 users upgrade to Firefox 4, which was launched last month to much fanfare (and many, many downloads).