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)


if (this.output.insertAdjacentHTML) {
        
        this.output.insertAdjacentHTML("BeforeEnd", message);
      } else {
        this.output.textContent += message;
      }
      if (message.lastIndexOf("\n") >= 0 && !this.disableScrolling.checked) {
        
        this.output.scrollTop = this.output.scrollHeight;
      }
    },
    _done: function Driver_done() {
      if (this.inFlightRequests > 0) {
        this.inflight.textContent = this.inFlightRequests;
        setTimeout(this._done.bind(this), WAITING_TIME);
      } else {
        setTimeout(this._quit.bind(this), WAITING_TIME);
      }
    },
    _sendResult: function Driver_sendResult(snapshot, task, failure, callback) {
      var result = JSON.stringify({
        browser: this.browser,
        id: task.id,
        numPages: task.pdfDoc ? task.lastPage || task.pdfDoc.numPages : 0,
        lastPageNum: this._getLastPageNumber(task),
        failure,
        file: task.file,
        round: task.round,
        page: task.pageNum,
        snapshot,