[pulvinar]

That's odd. ChatGPT (5.1) tells me Safari does not open an audio capture session for 'capability negotiation' that would show the mic icon.

I assume you have Safari > Settings > Websites > Microphone > facebook.com set to the default "Ask", or to "Deny"? Also check any installed extensions.

[35mm]

It's set to "Ask". I just checked again and Facebook is no longer activating the microphone. A glitch before maybe?