I have a full suspension bike with rear spring-style suspension.
I find the rear suspension too soft. How do I make it firmer?
With coil suspension you will need to purchase a new spring that is suited to your weight. Resistance comes from the spring so you need to purchase a heavier and thicker spring for harder suspension. There is usually a preload adjustment for fine tuning resistance but wouldn't change the feel drastically.
Most mid-high level suspension brands have a selection of springs (usually coloured for different weights) that you can purchase for different weight bands.
Air suspension allows you to adjust the pressure (volume) of air to change how soft it is.