Steps to Reproduce:

1. Visit http://sfr-front-end-development.us-east-1.elasticbeanstalk.com/ in Firefox on Mac
2. Inspect the typeface using the Fonts panel in the Inspector
3. Verify that system-ui is defaulting to Tahoma+Roboto Medium, although Firefox on Mac should have access to the Mac font stack.

I did some other digging and it would appear that Firefox will recognize San Francisco when adding -apple-system, BlinkMacSystemFont to the top of the stack.