What version of Firefox? Works for me. Did you get any of these pop-ups? see screenshots

Also, show the issue. Post screenshot showing that the camera has permission. Show the camera working in other browsers.

I have the same issue accessing the camera when trying to use Zoom in the Firefox browser. I attached photos showing Google Meet working in Chrome and not working in Firefox along with my camera permissions and the version of Firefox I'm running.

Are you running pro or home version of Windows? If pro, you should have sandbox enabled for testing. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

Any security software besides Windows Defender?

I'm running Windows 11 Home and only using Windows Security software.

Sounds like you may need to refresh Firefox. Make sure to back-up your data. I've been using Firefox for a few decades, but I've only done a refresh 1 or 2 times. https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Refreshing Firefox did nothing to help the problem. Exactly the same as before.

WOW. How about trying a different version. https://www.mozilla.org/en-US/firefox/channel/desktop/ Make sure to install in a different directory than your current install. Just add Dev, Beta, or Nightly to the path of the install. Also, how about providing system info if not DIY. Make and model?