Parts of apps
plugin-containerFirefox tab group
◐ Close the app instead
Runs a group of Firefox tabs, an extension, or its graphics.
What it is
Firefox’s helper processes. Each one hosts some tabs or a shared job.
If you close it
The tabs it hosts crash and ask to reload. Close the tabs or Firefox instead.
The verdict
◐ Close the app instead. A part of an app. Closing it alone breaks something, so close its app.
In Activity Monitor
plugin-container
From Procession’s guide, in the words the app shows.