Defaults to Tahoma on Mac Firefox

## 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.