Firm energy is energy you can count on.  Energy that you can dispatch when you need it, or that comes in a regular schedule that you can schedule energy balancing around. 

Capacity credits or payments are calculated in several different ways, and since the introduction of utility scale wind and solar energy generation the question of what 'Capacity Factor' means is by no means agreed upon.  Even a coal or gas steam plant will have some periods of maintenance, or have some risk of unexpected downtime.  Wind and solar extend this concept quite widely, where the capacity of a solar installation may reduce 50% in a matter of 30 seconds.  Photovoltaic solar arrays may be even more susceptible to sudden drops in production than wind farms, as sporadic cloud cover is even more difficult to predict than short term wind speeds.

When solar exists as only demonstration projects within a utility, representing only a miniscule fraction of total energy input, this issue is a small one.  As we come to actually rely on the energy produced from large solar arrays, the need to smooth out these sudden drops in production will become more important.