Since Firefox updated to version 52 anything else reliant on the Netscape Plugin API (NPAPI) is now forbidden. Which meansSilverlight, Java and Acrobat are no longer supported.
There is a workaround which involves downloading a special version of Firefox's latest browser. This version is called the Firefox Extended Support Release. This is available for Windows 32 and 64 bit, MAC OS and also Linux 32 and 64 bit operating systems
If you do a google search for it then you should be able to install it and Silverlight, Java and Acrobat will run, however please ensure you download the 32 bit version if using Windows as the 64 bit version does not work.