Attach (recommended) or Link to PDF file here:
[output.pdf](https://github.com/mozilla/pdf.js/files/6374987/output.pdf)

Configuration:
- Web browser and its version: Firefox 88.0
- Operating system and its version: macOS Big Sur 11.2.3 (20D91)
- PDF.js version: 2.8.243
- Is a browser extension: no/Firefox built-in

Steps to reproduce the problem:
1.  add Ink-Annotations to a pdf (i use https://github.com/highkite/pdfAnnotate, Annotations with appearance streams are shown correctly)
2. open output file in firefox

What is the expected behavior? 
The annotations should be visible.
![Bildschirmfoto 2021-04-26 um 09 15 48](https://user-images.githubusercontent.com/33630884/116043809-6a020f80-a670-11eb-973c-6f2df3dae532.png)

What went wrong? 
The annotations are added to the dom but invisible.
![Bildschirmfoto 2021-04-26 um 09 18 26](https://user-images.githubusercontent.com/33630884/116043867-7e460c80-a670-11eb-964d-0cc2c6dbfb61.png)

Tested successfull with FoxitReader, AcrobatReader, Chrome and Safari.
}
        const matrix = [1, 0, 0, 1, 0, 0];
        const bbox = [
          0,
          0,
          this.data.rect[2] - this.data.rect[0],
          this.data.rect[3] - this.data.rect[1],
        ];
        const transform = getTransformMatrix(this.data.rect, bbox, matrix);
        operatorList.addOp(OPS.beginAnnotation, [
          this.data.rect,
          transform,
          matrix,
        ]);
        const stream = new StringStream(content);
        return evaluator
          .getOperatorList({
            stream,
            task,
            resources: this._fieldResources.mergedResources,
            operatorList,
          })
          .then(function () {
            operatorList.addOp(OPS.endAnnotation, []);
            return operatorList;
          });
      }
    );
  }
  async save(evaluator, task, annotationStorage) {