bug(material-experimental/mdc-snackbar): JAWS does not read mdc-snackbar message in Firefox

[toastking]

Reproduction

https://stackblitz.com/edit/mdc-snackbar-bug?file=src/app/app.component.ts

Steps to reproduce:

1. Click on the button to open the snackbar with JAWS enabled

Expected Behavior

JAWS should read out the snackbar message of "Hello World" after the button is clicked.

Actual Behavior

JAWS does not read out anything.

Environment

• Angular: 10.2.3
• CDK/Material: material-experimental 11.0
• Browser(s): Firefox 81.0.2 (64 bit)
• Operating System (e.g. Windows, macOS, Ubuntu): Windows 10

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