A balloon full of helium has more mass than a balloon without helium, but less weight https://en.m.wikipedia.org/wiki/Mass_versus_weight.

~~Methane weighs more than nitrogen (70% of atmosphere); you lose weight and mass. Molar mass of nitrogen 7; methane 16. ~~

https://discuss.tchncs.de/comment/19776581

Edited to comment about mass: Except that a) methane is leaving the body and is not necessarily being replaced; the volume is simply taken up by a contracting colon and b) methane may be less dense than air, but it does contain fecal particles as well, which are tiny but still heavier than air.

Sharting makes you lighter for sure

Farts are mostly particulate. So, you’d be lighter.

Edit: I did not have “researching farts” on my dance card today, but here we are - I have inadvertently repeated a misnomer. While flatulence can be accompanied by aerosolized liquid, microbial elements, and particulate fecal matter, our gut is actually much more efficient than previously thought at separating gasses. Healthy farts do not contain a significant amount of non-gaseous material. A tiny bit of such material would weigh more than the gaseous content, hence the misnomer that farts are mostly liquid and solid material by weight.

As the gasses contain hydrogen and sometimes methane, they are indeed lighter than air, on average. They may also contain sulfides and carbon dioxide that are heavier than air, but in smaller quantities.

So, according to flatus experts (yes, that’s a thing), yes, farting on Earth likely makes you heavier.

For those who want to suffer as I have, here is a fairly comprehensive article on the matter.

As a homogenous blob, you would become lighter and denser.

I think, you get lighter from the thrust, and the loss of those rarefied substances also reduce your mass.

Methane is lighter than air, so farting makes you heavier.

But not in any measurable way.