At Mozilla, we work hard to make Firefox the best browser for you. That’s why we're always focused on building a browser that empowers you to choose your own path, that gives you the freedom to explore without worry or compromises. We’re excited to share more about the updates and improvements we ha...
In the specific case of PDF most users wouldn’t even know where to add an alt text. Depending on how you generate the PDF it might even be impossible. So I think Mozilla has the same concern as you, and that’s why they aren’t adding this to images in HTML (yet).