Attach (recommended) or Link to PDF file here:
https://guichet.public.lu/dam-assets/catalogue-formulaires/tva/tva-option-aic-pers-morales/tva-option-aic-pers-morale-DE.pdf

Configuration:
- Web browser and its version: Firefox Nightly 91.0a1 (2021-06-26) (64-bit)
- Operating system and its version: Windows 10.0.19043.1052
- PDF.js version: N/A

Steps to reproduce the problem:
1. enableXFA in Firefox Nightly
2. Open form

What is the expected behavior? (add screenshot)
Header image is scaled properly
![image](https://user-images.githubusercontent.com/7104454/123525954-905d1f00-d6a2-11eb-840e-be30eee395e1.png)

What went wrong? (add screenshot)
Image is scaled to fit
![image](https://user-images.githubusercontent.com/7104454/123525957-96530000-d6a2-11eb-9991-bf7f37c6f1a5.png)


warn("XFA - Something goes wrong: please file a bug.");
          return mainHtml;
        }
      } else {
        hasSomethingCounter = 0;
      }
      targetPageArea = null;
      const page = pageArea[$toHTML]().html;
      mainHtml.children.push(page);
      if (leader) {
        page.children.push(leader[$toHTML](pageArea[$extra].space).html);
        leader = null;
      }
      if (trailer) {
        page.children.push(trailer[$toHTML](pageArea[$extra].space).html);
        trailer = null;
      }
      const contentAreas = pageArea.contentArea.children;
      const htmlContentAreas = page.children.filter(node =>
        node.attributes.class.includes("xfaContentarea")
      );
      hasSomething = false;
      const flush = index => {
        const html = root[$flushHTML]();
        if (html) {
          hasSomething =
            hasSomething || (html.children && html.children.length !== 0);
          htmlContentAreas[index].children.push(html);
        }
      };