Many web pages contain links to documents of various types including Adobe PDF documents, and other types of files such as videos. Firefox allows you to view those links embedded within a Firefox web page. This requires use of a plugin called mozplugger whichprvides this embedding capability.
Your system administrator can install this plugin, if it is not already installed. If you have Ubuntu or root access in other distributions such as Fedora, you can also do this yourself.
In any distribution, open a shell or terminal session.
In Ubuntu use the following command to install the plugin.
sudo apt-get -y install mozplugger
In Fedora or most other distributions, su - to root, then use the following command to install the plugin.
yum -y install mozplugger