IPA: /ˈfɜːrnəs/
KK: /ˈfɜrmnəs/
The quality of being strong, solid, or stable; the ability to remain unchanged or to resist pressure.
The firmness of the table made it very sturdy and reliable for heavy items.
Firmness → It is formed from "firm" (from Latin "firmus", meaning strong or stable) and "-ness" (a suffix indicating a state or quality). The word "firmness" refers to the quality or state of being strong or stable.
Think of the word 'firm' which means strong, and remember that 'firmness' is the quality of being strong or stable.