mostly it is used in web-browser.It is also known as add on.
mostly we use plug ins in web browser for video streaming purposes such as we use adobe flash player to watch videos and games ad -blocker .
The browser developer builds the web browser which will works in minimum requirement at first. minimum requirement means it will be able to play the web content in the web browser.
if the user wants to add some extra functions to his/ her browser then such kind of person needs a plug-in to make extra functions.
plug ins enables support the third party softwares & helps to extend the new feature that you want in your software. plug ins are used to play videos, play games, decrypt and encrypt emails etc.
if you need your browser to work some extra functions then this could be very helpful link to download the plugin for mozilla firefox browser.
Here you can find appropriate plugins for your mozilla browser and can download it.