NOTE: Usually concrete or physical usages
The Jell-O never firmed up, even when bordered by beautiful lettuce.
Like other muscles they can be firmed up by exercise.
The loose teeth had firmed, the gums have a pink, healthy color.
WordNet 3.0 Sense Numbers: 1, 2
NOTE: Usually abstract or metaphorical usages
We have firmed up plans of entering power and energy industries.
The challenge has only firmed up her resolve.
So I lied about the beanpot schedule; it never really firmed up, and I am slow to update.
WordNet Verb Particle Constructions, Multiword Expressions: