Scene description is read twice on Firefox NVDA #520

The Scene description is read twice on Firefox on Windows with NVDA. The Scene description is only read once (expected) on other browser and screen reader combinations that we have tried.

To reproduce:

1. Use NVDA’s heading navigation (press 'h') to get to the "Scene" H2 heading
2. Use NVDA’s content browsing to read the document content following the "Scene" heading (press the down arrow)

Expected:

1. "Scene heading level 2"
2. "Graphic <scene description>"

Actual:

1. "Scene heading level 2"
2. "<scene description> Graphic <scene description>"

Original Jira issue: https://fluidproject.atlassian.net/browse/C2LC-392