"Would firm generators facilitate or deter variable renewable energy in a carbon-free electricity system?"

Economic incentives to run high-capital-cost technologies with high capacity factors puts nuclear in direct competition with variable renewables.
https://www.sciencedirect.com/science/article/pii/S0306261920312733

In every simulation we did, putting more nuclear on the system leads to building less wind and solar and building more wind and solar leads to building less nuclear.

We have never seen a case where building more nuclear facilitates more wind and solar.

(1/2)

The idea that nuclear facilitates wind and solar by providing backup when the sun doesn't shine or the wind is not blowing does not seem to cost out.

If you have the nuclear, why would you build the wind turbines?

[Solar correlates with demand, so has added value.]

(2/2)

I should note that people are coming up with designs coupling nuclear power with heat storage, with short-term generating capacity substantially exceeding the magnitude of the sustained heat source.

The economics of this "dispatchable" nuclear could be different.

By the way, a slightly more recent version of the model used in that paper is available here:

https://github.com/carnegie/MEM_public/tree/Henry_et_al_2020

As I point out elsewhere in this thread:

There are many conditions under which nuclear, wind and solar all compete in a least-cost electricity system.

These technologies will compete in a market and the success of one will come at some expense to the others.