oppia: Auto-generated English audio not available in Firefox

Describe the bug There is no option to play auto-generated English audio in Firefox (this option does appear when testing in Chrome).

To Reproduce Steps to reproduce the behavior:

1. Go to the first Fractions exploration - ‘What is a Fraction?’
2. Open the voiceover menu
3. Click on the dropdown to see the list of available options

Observed behavior There is no “English (auto)” option in the dropdown menu.

Expected behavior This option should appear in the dropdown menu.

Screenshots

Desktop (please complete the following information; delete this section if the issue does not arise on desktop):

• OS: macOS
• Browser: Firefox
• Version: release-2.9.2