On Firefox 44 and later, some Web fonts may be rejected due to the stricter validation by a new version of the OpenType Sanitizer. Fonts containing OpenType layout tables that are violating the spec will not be rendered and the Web Console shows a warning about the error.
Update: Due to a couple of affected sites including Nokia, the validation has been loosened on the Beta and Release channels since Firefox 45. However, problematic fonts should be replaced anyway.
Update 2: Some fonts on Google Fonts are also broken, showing “tofu” characters on Firefox Nightly and Developer Edition.
Update 3: The broken fonts on Google Fonts have been fixed by Google in October 2017.