I've been doing a bunch of testing these last few days to confirm this issue.
You can see this issue by creating a new Foundation site with the Zurb template. Run the NPM start task, and preview in the latest versions of Chrome, IE and Firefox (in Windows).
Chrome and IE has font sizes that are fairly close to each other.
Firefox has body font sizes that are slightly larger, and header font sizes that more than 2 times smaller, than the other browsers.
I've reset all modifications to my Firefox settings, so it is not something weird regarding zooming.
I'm going to try this at home on my Mac to see what happens in Chrome, Safari and Firefox. I'll add to the thread if the same issue happens there.