Obsidian Fireheart

The ability gained by the land triggers at the beginning of the upkeep of the land’s controller, not the beginning of the upkeep of Obsidian Fireheart’s controller. The player who controls the land at the time the ability triggers is the one who’s dealt damage.

As the reminder text indicates, whether the targeted land has the triggered ability that’s been granted to it depends only on whether it has a blaze counter on it, not on whether Obsidian Fireheart is still on the battlefield.

A land with a blaze counter on it is an illegal target for this ability. You may activate the ability targeting a land, then activate the ability in response to itself targeting the same land. Although the first ability to resolve will put a blaze counter on that land, the second ability to resolve won’t resolve.