A subject close to my (namesake) heart! Flatulence is gas molecules, so of course it has mass and weight. “Intensity” is some combination of concentration, pungency, and unpleasantness. “Aroma” basically amounts to chemistry.
The less smelly farts are largely composed of swallowed air, which has an effective molecular weight of 29 daltons. Some people habitually swallow large amounts of air. The “rotten eggs” smell indicates hydrogen sulfide gas, H2S, with a molecular weight of 34. Also issuing from the colon is methane gas, CH4, with a m.w. of 16.
As you can see methane is lighter than air while H2S is heavier. There are lots of other stinky substances synthesized by bacteria in your gut, such as mercaptans, indoles, skatoles, and sulfides. There are people with PhDs in organic chemistry who study such things with mass spectrometers and gas chromatographs. Almost all of these substances are larger molecules whose gas would definitely be denser than air. In that sense weight is proportional to concentration or “intensity” but I’m not sure the density / buoyancy matters much, because low concentrations and natural convection currents will keep these gases well mixed and dispersed through the space in which the fart is confined. In still air there is still the process of diffusion, whose rate (as I recall) is proportional to square root of molecular weight. Under the right conditions the stink creeps slowly.
All of these gases area toxic in high enough concentrations (people commit suicide with methane, aka natural gas) but due to dilution in room air, the concentrations are probably too low to be toxic no matter how disgusting the smell. In a closed space, such as a closet, I suppose, in the extreme, a large volume of flatulence could be dangerous. People with bad flatulence should adjust their diets, come out of the closet, and stay downwind.