What error message are you receiving?

I don't get any error message at all.

Could you elaborate on how you are not able to install then?

Which Silverlight plugin version are you trying to install?

Some Silverlight versions are blocked on Mac.

• https://addons.mozilla.org/en-US/firefox/blocked/

You may have an older Mac 10.6.x version, so make sure that you have updated to OS X 10.6.8

I close out Firefox. I download Silverlight and install it. I open Firefox and it does not show up when I click on add ons or look under the "Troubleshooting info." I had trouble installing the last version, too. I had to delete the pluginreg.dat file and then relaunch Firefox in order to get it to work. Then, recently, it stopped working, so went I looked in the plug in list, there was an alert from Firefox that the application had been blocked and I needed to upgrade it. As soon as I downloaded the newer release of Silverlight, it refused to install again and deleting the pluginreg.dat file did not work this time around. I received another notice after yours that Firefox is blocking all versions of Silverlight 5.1 and 5.1.2. Which would pretty much explain the problem. Not even the folks at Microsoft are aware of this as I spent over an hour having them walk me through trying to install with no luck. I will have to use a different browser that is more compatible with this plug in.

Well, that explains it. Thanks for letting me know. It works with other browsers so I will have to use another.

FYI, all of my software is up to date, so that wasn't the problem.

I've also been unable to install the Silverlight plugin for Firefox 19.0.2. on my macbook pro (using Mac OS X 10.6.8). I found it convenient to simply use my Safari browser, which has never failed me.