Old versions plugins always present security risks and stability issues for any browser, Mozilla is blocking these types of plugins to protect users from using them, you can always check updates for your plugins using Mozilla plugin check, install them if the update is found and suggested by the page. Mozilla announced today in its add-ons blog that they have added older versions of the Silverlight plugin for Mac OS X to their block listIf you are a Mac user using Silverlight plugin version earlier than v5.1 in Firefox, it is recommended to disable it immediately.
You can visit the plugin verification page provided by Mozilla in the Firefox browser to check for the updated version of Silverlight or download the latest version of Silverlight website.
Silverlight plugin blocked by Mozilla for causing stability issues like crashing Firefox on popular sites like Hotmail. So if you are experiencing crashes in Firefox, first download and install Silverlight 5.1 on your computer.
This plugin will be automatically disabled by Firefox, but you can enable and use it at your own risk, which is not recommended.