Firm power is electricity generation that is guaranteed to be available 100% of the time, except for scheduled maintenance, and is used to meet electricity demand at any time, even under adverse conditions. It is a reliable, dispatchable power source that can be contracted or sold as a commercial attribute in electricity markets, contrasting with intermittent power like solar and wind. Clean firm power refers to carbon-free resources with these same reliable characteristics, such as geothermal, nuclear, and long-duration storage.
– source: Google  AI Overview

Here are two articles that talk about firm power:

Brookfield says hydro and nuclear are back in demand as baseload PPAs shift to firm power
November 10, 2025
By Kevin Clark
www.renewableenergyworld.com/news/brookfield-says-hydro-and-nuclear-are-back-in-demand-as-baseload-ppas-shift-to-firm-power

Clean Firm Power
cnee.colostate.edu/wp-content/uploads/2025/01/Clean-Firm-Power_Final.pdf