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)


const parent = this[$getParent]();
    if (parent instanceof Field) {
      if (parent.ui && parent.ui.imageEdit) {
        if (!this.image) {
          this.image = new Image({});
        }
        this.image[$content] = value[$content];
        return;
      }
    }
    const valueName = value[$nodeName];
    if (this[valueName] !== null) {
      this[valueName][$content] = value[$content];
      return;
    }
    
    for (const name of Object.getOwnPropertyNames(this)) {
      const obj = this[name];
      if (obj instanceof XFAObject) {
        this[name] = null;
        this[$removeChild](obj);
      }
    }
    this[value[$nodeName]] = value;
    this[$appendChild](value);
  }
  [$text]() {