Plugins are loaded separately from Firefox, allowing the main Firefox process to stay open if a plugin crashes. This article explains what plugin-container is for.
What is a plugin?
A plugin is a piece of software that displays Internet content that Firefox is not designed to display. These usually include video, audio, online games and presentations that are made in proprietary formats. Plugins are created and distributed by the companies that make those proprietary formats. Some common plugins are Adobe Flash, Java and Silverlight. For more information on plugins, see Use plugins to play audio, video, games and more.
What is plugin-container.exe?
Each plugin is loaded separately from Firefox in a plugin-container.exe process, allowing the main Firefox process (firefox.exe) to stay open if a plugin crashes. There are as many plugin-container.exe processes as plugins launched since the Firefox session startup. For more information on plugin crashes, see Send plugin crash reports to help Mozilla improve Firefox.
If some plugin-container.exe processes use a lot of computer resources, see Firefox uses too much memory or CPU resources - How to fix.