Another long-running issue on my dev machine solved today. I never got the Silverlight plugin to work in Firefox. With every upgrade of either Firefox or Silverlight I hoped that the issue was solved, but no. Today, I finally found the solution here. Yes, it really was that simple.

When going to the Microsoft Silverlight page at microsoft.com/getsilverlight, it would always nicely report that it was installed and had the newest version. Also, browsing about:plugins would report everything being in order. However, no pages using Silverlight (like nos.nl that features a Dutch online news and sports tv channel) and more recent, this bytes.com interview with my hero Anders Hejlsberg) would ever work. They would simply remain blank or show a banner that links to the silverlight install page… which would tell me it was already installed. Of course, I could quickly Show this page in IE, but it bugged me nonetheless.

I tried different configurations, suspected a clash with this or that SDK and uninstalled and reinstalled both Firefox and Silverlight multiple times. As I found out today, it was just a minor detail in my computer’s configuration… A change in the configuration I think should have been made by the Silverlight installer: it fails to add the path to the Silverlight installation to my system’s path. This is needed for Firefox to find the plug-in when it needs to run. I didn’t know that until I found the forum thread on silverlight.net.(My path is fairly lenghty due to the presence of a multitude of developer tools, might have something to do with that.) Why it worked in IE, I don’t know, perhaps in there the plug-in is installed with a full path specification instead of relying on the system path. It is also strange why pages like microsoft.com/getsilverlight could detect the plug-in’s presence but was not able to run it.

Solution: (on Windows 7), with a user that has administrative priviliges, do this:

• Right click “Computer”, choose Properties
• In the left column, click Advanced system settings
• Go to tab Advanced
• Click button “Environment variables …”

The top half will probably display your user account’s personal path, and will probably already contain an item that is called PATH. If it is not, add it, if it is, edit it.

At the end of the value (if there was already something there, separate it by a semicolon ; ), add the following:

C:\Program Files (x86)\Microsoft Silverlight\5.0.61118.0

Click OK until all dialogs are confirmed.

Restart your browser to be sure, and Silverlight now works in Firefox!

Note that this version number might change with future updates of Silverlight, so you should be prepared to edit it after an update if the Silverlight installer fails to do that. Check your own computer’s file system for the correct path.

Ad 22-10-2013. As it turns out, it is not so much the path to Silverlight missing, as it is being present but incorrect. If you have a newer version on your system but the path points to an older version, the detection will work but FF-Silverlight will not. Instead of updating the path after every update, as I did before, simply remove it from the path. I did, and at least on my system, FF-Silverlight is still working.