The plugin container is used to run plugins separate from other browser code. The most common would be Flash. Some sites use Flash for communication, so those sites may ask for microphone permission. I'm pretty sure you control whether any particular Flash media can access the microphone, even if plugin-container.exe has 24x7 access. To check the sites that currently have access, see:

https://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager06.html

(You need to approve the site to use Flash in order to see the list; this site is owned by Adobe.)

Thanks for the reply. In practice I have no reason to worry. It's a normal thing. The microphone will only be used when I decide to use it, right? Thanks again.

Yes, the default setting for Flash is that every site needs site-specific permission to use the microphone or camera. You also an set a global "deny" preference. There's no global "allow" option.

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

Thanks for solving my doubt. Now I'm more quiet about Firefox's use of the microphone. Best regards, Felice.