Doesn't work anymore. Just installed this extension, loaded the SF page, and clicked on the extension. Nothing happened.

does not work at my custom-domain setup like domain.my.salesforce.com, but works great at my dev org.

This appears to work on the first 10-12 fields, and only intermittently on the rest of the page layout. Sometimes a lower field will show the API name when I turn this off and then on, but mostly the lower fields display the labels. I looked over the fields further, and the API names that show on the Page Layout are not the API name for the referenced field. Possibly having same labeled fields confuses this extension. Uninstalled!