Why was the Fire Nation so indifferent about the Northern Water Tribe after conquering the Earth Kingdom?

That's because the Northern Water Tribe wasn't a real threat, The (broken yet still active) Earth Kingdom was.

In TLA we can see the entirety of the Northern Water Tribe which is practically speaking 1 city with probably some small settlements scattered across the North Pole. The Earth Kingdom on the other hand was by far the largest and militaristic enemy the fire nation has faced. (Seeing the air Benders did not have a standing Army as Aang mentioned while visiting a fire nation school).

Now compare both conflicts:

• The Northern Water Tribe was stuck on the north pole in a pure defensive position. Sure the Fire Nation was unable to breach their defenses but they had them contained seeing the Northern Water Tribe was either unable or unwilling to launch a counter attack.
• The Earth Kingdom still had (mostly)free reign across the largest landmass in the world (roughly 4 times the size of the Fire nation lands). And as seen in several episodes during the Ba Sing Se story arc they do have a capable military leadership and noteworthy standing army. If it weren't for the Dai Li meddling with the state affairs the Earth Kingdom might have been able to push back the Fire Nation.

This doesn't mean that the Northern Water Tribe would not eventually become a target, but until the Earth Kingdoms and, most importantly, it's people had been subdued it will be not their highest priority. This is because although the Earth Kingdom had been defeated by the fall of Ba-Sing Se it would not mean that the civilian population (or the military for that matter) would accept this. Seeing the Earth Nations culture this would probably create a warlord like state (like what happened after the Queen's death in Legend Of Korra). Without swift action these warlord states would become more organized and would probably eventually band together against the Fire Nation, making the fall of Ba Sing Se a hollow victory in the long run.

So yea, The Earth Kingdom was "officially" defeated the war would continue on against rebel (civilian) groups and former Military command chains in the form of warlords. Once these have been dealt with (using Sozin's comet to decimate any remaining resistance and breaking the rebellious spirit of the Earth Kingdom people), they would probably focus on the Northern Water Tribe. But until then the Northern Water Tribe and their defensive posture would not be a threat to the Fire Nation or it's policies.

The greatest Strength the Fire Nation had was it's unity. The entire nation fighting as one. While the Earth Kingdom, that although had a unifying government was fractured and disorganized with the Water Tribes even being separated. This gave the Fire nation opportunities to pick them off one by one. Launching another full scale attack on the Water Tribe might push them to unify more with the remnants of the Earth Kingdom(don't poke a sleeping bear when wrestling with an aligator).