1999 Mercury Sable car at idle

There are a lot of possibilities that can cause issues at idle. The first things to check are the cap and rotor, (unless your car has a coil box and does not use a distributor), plug wires and plugs. If there is any question, you should replace them all because they are part of a regular service anyway.
Then there are electrical items that might be at fault. That is part of the challenge with electrical stuff as it will fail slowly, intermittently and in all cases, totally. It can also be an issue where one weak part makes another fail. Overheated electrical will put demands on dependant components and can damage them. I have fixed stuff and had other stuff fail at a later date due to aforementioned situation. So, it is best o go through a process of elimination, but use reason when a part failure can damage other parts when it catastrophically fails.
Before you replace anything, let me know how the plugs look.
Pull the plugs when you get a chance and look at the tip. A brownish tan color is normal. If you find anything else. Let me know what it looks like.
The next step will be running a can or 2 of injector and intake tract cleaner. This is the issue for many fuel injected cars having performance an idling problems. I use BG44K as it is the only cleaner that I have found that truly cleans the injectors and intake tract to the pint where even injectors near failure can be saved. You might have to go to a service station or car dealership to find it as auto parts store do not always carry it.

If the diagnostic shows that the O2 is bad it is something to look at. Usually the cxar will start to get bad gas maileage when the O2 is bad. But other things can happen and it is a regular service item.
I would not clean a mss air flow sensor becuase they arte easy to damage3. Even when you replace one you have to go to a dealer and have them reflash the ECU to recognize the new MAF. Soetimes people coinfuse it with a MAP that is easier to clean.
Shange the O2 and see wht hapeens.

The mass air flow sensor is going to be part of the intake path befor the throttle boady and will have an electrical lead coming off of it. It is very delicate because it has 2 very thin wires that are crosshaired in the middle off the air intake. There is a sparay cleaner available but they tend to do much more damage than fixing anything.
The O2 sensor is part of the exhaust and will be in between the exhaust manifold and the catalytic converter. It will also have an electrical lead coming off of it.
The O2 sensor should be replaced every so often as part of a complete service when higher mileage items are included in maintenance. So, between the diagnostics, the behaviour of your car and what my first instinct is, you certainly won't waste any money replacing the O2 sensor.
It is going to be one less thing to worry about either way. I do think this will fix it or in the least will be part of the path to fixing it.
Let me know the outcome and I will keep the post link on top of my list so I can replay ASAP.
I will be moving this week, so it might take me a little longer than usual to get back to you.