I ran into the problem with Firefox 5 where when I visited a web page using Silverlight that Firefox prompted me that the plugin would not work and I would need to restart in 32-bit mode. After the restart, I would work but its a pain as each time I restarted I would be prompted to again.
As it turned out all I had to do was update my version of the Silverlight plugin as the problem had been fixed. First I uninstalled Silverlight on OSX it quite straight forward. I followed the instructions from Microsoft.
Removing Silverlight plugins on Macintosh
- Access your hard drive
Double-click the hard drive icon on your desktop
- Find the plugin
Navigate to your Internet Plug-Ins directory: /Library/Internet Plug-Ins/
- Remove the plugin
Drag any of the following into your trash bin:
After removing the old version I went to the Silverlight install page
and downloaded the latest version and ran the installation program.