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)


);
          this.canvas.mozOpaque = true;
          ctx = this.canvas.getContext("2d", { alpha: false });
          task.pdfDoc.getPage(task.pageNum).then(
            function (page) {
              var viewport = page.getViewport({ scale: PDF_TO_CSS_UNITS });
              self.canvas.width = viewport.width;
              self.canvas.height = viewport.height;
              self._clearCanvas();
              
              var renderAnnotations = false,
                renderForms = false,
                renderPrint = false,
                renderXfa = false;
              var textLayerCanvas, annotationLayerCanvas;
              var initPromise;
              if (task.type === "text") {
                
                textLayerCanvas = self.textLayerCanvas;
                if (!textLayerCanvas) {
                  textLayerCanvas = document.createElement("canvas");
                  self.textLayerCanvas = textLayerCanvas;
                }
                textLayerCanvas.width = viewport.width;
                textLayerCanvas.height = viewport.height;
                var textLayerContext = textLayerCanvas.getContext("2d");
                textLayerContext.clearRect(
                  0,
                  0,
                  textLayerCanvas.width,