Old versions of the Silverlight plugin for Mac OS X have been added to the Firefox add-on blocklist. We did this because these versions of the plugin are causing instability in Firefox, including crashing on popular sites such as Hotmail.
The affected versions – all versions older than 5.1.0 – will be disabled. To avoid the block, we recommend that you visit the Silverlight page and update your Silverlight plugin to the latest version. If you are unable to update it, or for some reason you need to use the a blocked version of the plugin, it can be enabled again in the Add-ons Manager, in the Plugins pane.