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)


while (document.styleSheets.length > 0) {
        const styleSheet = document.styleSheets[0];
        while (styleSheet.cssRules.length > 0) {
          styleSheet.deleteRule(0);
        }
        styleSheet.ownerNode.remove();
      }
      const body = document.body;
      while (body.lastChild !== this.end) {
        body.removeChild(body.lastChild);
      }
      const destroyedPromises = [];
      
      for (let i = 0; i < this.manifest.length; i++) {
        if (this.manifest[i].pdfDoc) {
          destroyedPromises.push(this.manifest[i].pdfDoc.destroy());
          delete this.manifest[i].pdfDoc;
        }
      }
      return Promise.all(destroyedPromises);
    },
    _exceptionToString: function Driver_exceptionToString(e) {
      if (typeof e !== "object") {
        return String(e);
      }