Firefox should detect the default microphone and camera. There is a way to change it, this was the bug to fix this https://bugzilla.mozilla.org/show_bug.cgi?id=804611

Please also check the settings here: https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues#w_...

The audio preferences are in the Media Mixer in Windows 7, the stored cookie may still have a previous preference if it was recently changed. Though this would be strange.

Go to the web page. Once the page is loading, mouse to the address bar and Left click the icon. A window for displaying site information should come up. Select More Information. Now select Permissions. In the menu, find and check the plugins, make sure they are set properly.

Is this with using a plugin like Flash?

In that case you can try to reset the Flash local storage and also check the permissions in Flash.

• Windows Control Panel > Flash Player > Storage: Local Storage Settings
  • Remove a specific domain: Local Storage Settings by site
  • Remove all domains: Delete All and Delete Data

Flash Website Storage Settings panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

Global Storage Settings Panel:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html

Flash Global Privacy Settings:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager02.html

See also Flash Privacy settings:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help09.html