So, if you go to any sites that have been designed for this new Silverlight stuff, you can download the plug-in from here (but make certain that you are downloading SilverLight v.1.0 for OS X (10.4.8 upwards) if you are using a PPC Mac, but even this will not work with Safari 4. Version 2 only works with Intel Macs and does work with Safari 4/5. Download the current version of Silverlight. Double-click on Silverlight.dmg in your browser's Downloads menu. For Safari, the Downloads menu is in the upper right corner of the browser. For Firefox, Downloads is a separate window located in the Dock. Double-click on Silverlight.pkg. Follow the installation prompts.
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.
Microsoft Silverlight Download
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.