Time to put your singular, partially functional brain cells to use.
Don't be silly! Singular things can't be plural. Singular brain CELL. Anyhow, ad-hominem is across purposes to learning and communication and is best left out. It is the last resort of the intellectually weak; they attack the thinker because they are unable to attack the thought. Viciously attack the thought, never the thinker!
How do things on Earth fall down without gravity
There are many ways to conceptualize the answer. They fall because they were lifted (and with only the energy that was used to lift them). They fall to return to their "lowest energy state". They fall because the less dense media below them cannot support their weight.
Density is the amount of matter packed into a certain space.
True, but many times when flat earth researchers talk about density, they are talking about weight density. The "force", is weight! This is the effective weight; the weight with buoyancy factored in - as opposed to the intrinsic weight which does not factor that in.
Neither are forces.
Bouyancy is absolutely, explicitly, a force; As is weight!
Why doesn't it fall up?
Because that would be silly, and because it doesn't. A lot of the things we know about the world come purely from observation. In science we call them natural laws, and we rarely get deeper insight into why/how. They are often "scientific bedrock", and the manifest cardinal directions of up and down are no more explained or explainable than the spatial dimensions themselves or the matter we are discussing the behavior of! They are, and we have to deal with it!
Things fall down in a vacuum, where there is no air.
There is no such thing as a vacuum, just a lower pressure. There is always air in the chamber, and even theoretically - there can't NOT be.
As for buoyancy, that's just saying gravity again.
Only when you misunderstand what is being said (which is REALLY easy to do, in part because of vernacular). There is no gravitation (gravity is a natural law millenia old, and probably isn't going anywhere). Weight is an intrinsic and inexorable property of all matter. It is not imbued by magic omnipresent "fields" that defy logic, rigorous description, measurement, and manipulation. Wether something falls (gravity), stays put (neutrality) or rises (levity) is primarily to do with the interplay of the volumetric WIEGHT density of the object and that of the surrounding media.
Helium balloons float up because they are lighter than the air.
Almost. They float up because they are lighter than the volume of media they displace.
Not less dense.
Actually, it's both. Like the conceptualizations above for the "reason" things fall, there is more than one way to do so and come to a rational understanding.
We can say that the balloon rises because it weighs less than the surrounding media it displaces, and be correct. We can also say that the helium balloons density (its volumetric density!) is lesser than the media it displaces (the surrounding media), and be correct too!
A lot of confusion is created when people use vernacular that has a different meaning outside the discipline than within it. It happens in almost every branch of science/philosophy and mathematics. Hopefully I have cleared up some of the confusion caused by the terms being used. If not, ask some more questions!
You can also say that Earth is accelerating upwards to keep things on it. But, what is causing that?
Presumably the same infinite and inexhaustible violation of the law of conservation that powers "gravitation's" omnipresent acceleration of all matter simultaneously... I agree it is hard to swallow; which is why I, personally, do away with the whole business.
Why haven't we hit the speed of light yet (which is impossible by laws of physics, which none of you have any grasp of)?
Just getting started it is hard to see or believe, but keep studying the subject and you will find that knowledge of physics and the history thereof is what leads to ideas like UA and many other aspects of flat earth research. The lack thereof is what encourages to simply repeat what we are told, and worse - believe it.