Well, assuming fire temperature to be 800C and temperature drop off close to 1/r² - you need to be approximately 28 meters away from a point source to feel 1 degree Celsius increase in temperature. Forest fire is not a point source, it’s a wall of fire, but give it or take - 100 m away it you shouldn’t feel any heat coming from the first fire.

It was a way to support nice and friendly website back then, sort of like donations

I would think that updating 50.000 Lemmy instances would have a similar performance effect of 50.000 Lemmy users browsing one instance. In that case instance with one user won’t benefit the Fedeverse, but shouldn’t be an issue either.