I understand this as you saying it is a site specific issue? Then it is not a bug.
I can’t guess what you know about HTML, CSS and page rendering - so I will explain - in general terms what happens when you open a document in a browser.
All fonts used in a document usually defines one or more fonts defined directly in the document or using CSS.
If a specific font is used on any given webpage Firefox will render the font if available on the user’s system.
If the specific font is not available Firefox will pick one from the same family and if that fails Firefox will use what it think is the best alternative.