Directly from "firm":
* affirm: to state positively; declare to be true
* confirm: to make certain; verify
* infirm: weak; not strong
* unfirm: not firm; shaky; uncertain
Derived from "firm":
* firmly: with firmness; resolutely
* firmness: the quality of being firm; solidity; strength
* affirmation: a positive statement; a declaration
* confirmation: proof; verification
* infirmary: a hospital or place for the sick
* affirmative: expressing agreement or approval
Other related words:
* ferment: to cause to undergo a chemical change, often with bubbling or frothing
* firmament: the sky or heavens
* affirmative action: policies designed to promote equal opportunity for members of historically disadvantaged groups
These are just a few examples, and there are many other words that share the root word "firm."