Non-LTS versions of Ubuntu only receive security updates for up to 9 months after their release (for Ubuntu 15.10, until 07/2016) and are not recommended for use after the expiration of the support.
Would there be an imminent threat for a user, who is generally careful (not adding third-party repositories, using a web browser without JavaScript/NoScript, not executing unknown files/commands, active software firewall) in continuing to run an unsupported version for a few more months (e.g. 3-4 additional months until the release of the next equal month-version, such as .10 to .10)?
If yes, what would be the most likely causes of a security breach?