Heading in aria-live area or with aria-live itself announces twice in Firefox

[joppekroon]

Steps to reproduce:

Start with following HTML code:

<button>click me</button>
<h1 aria-live="polite">Page title 1</h1>

<script>
const button = document.querySelector('button');
const heading = document.querySelector('h1');

let index = 1;

button.addEventListener('click', () => {	
	heading.textContent = "Page title " + ++index;
})
</script>

or

<button>click me</button>
<div aria-live="polite"><h1>Page title 1</h1></div>

<script>
const button = document.querySelector('button');
const heading = document.querySelector('h1');

let index = 1;

button.addEventListener('click', () => {	
	heading.textContent = "Page title " + ++index;
})
</script>

Click the button to change the content of the heading and trigger the live area announcement.

Actual behavior:

On Firefox the changed content of the heading is repeated.
NVDA Speech: "Page title two Page title two".

Expected behavior:

NVDA announces the changed content of the heading once also on Firefox.
NVDA Speech: "Page title two".

NVDA logs, crash dumps and other attachments:

nvda.log

System configuration

NVDA installed/portable/running from source:

installed

NVDA version:

2024.3.0.33433

Windows version:

Version 10.0.19045 Build 19045

Name and version of other software in use when reproducing the issue:

Firefox 129.0.2 (64-bit)

Other information about your system:

None

Other questions

Does the issue still occur after restarting your computer?

Yes

Have you tried any other versions of NVDA? If so, please report their behaviors.

No

If NVDA add-ons are disabled, is your problem still occurring?

Yes (No add-ons installed)

Does the issue still occur after you run the COM Registration Fixing Tool in NVDA's tools menu?

Yes