I tried running Silverlight 5.1 on Firefox 39.0 for Ubuntu via Pipelight plugin. Pipelight plugin has installed Silverlight successfully and it's shown as enabled when I type `[sudo] pipelight-plugin --list-enabled`. It worked for more than a month until today and I am still not sure why. I reinstalled Pipelight multiple times and I am completely sure there are no problems with it. I am using User Agent Switcher but I also tried other UA switchers and they all work fine. I tried running some simple tests such as http://bubblemark.com/ and they prompt me to install Silverlight. I was thinking what is a problem and then I check about:plugins page. There is no Silveright on that page. :| I tried refreshing the Firefox, re-initializing the plugin database (as shown in official help for Firefox) and running `sudo pipelight-plugin --create-mozilla-plugins`. Pipelight has finished it with no problems, but there still isn't a Silverlight plugin in about:plugins.