So, I was having trouble even running the package manager, because of "guardrail-agent" (which I think is installed by default on Google Cloud images). To resolve this, I did:
sudo apt-get remove scriptrock-guardrail-agent
After that, I was able to install the needed package:
sudo apt-get install libasound2
Then, Firefox was able to run (as long as Xvfb is running, which was managed by Jenkins).