Due to some performance issues I was having with Firefox and sites with a lot of Flash, I decided to give Flashblock a try. I’m not necessarily a proponent of blocking advertisements - this blog uses ads to pay for the bandwidth - however some of my favorite sites were causing some downright unacceptable sluggishness in Firefox. I eventually narrowed the problem down to the many Flash ads some sites put on the page (at the time of writing this, Engadget has six Flash advertisements on their main page).

To my disappoint, however, Flashblock has the unintentional side effect of blocking Silverlight applications. I don’t think the addon is explicitly blocking them. Instead I believe Silverlight simply uses some syntax that Flashblock blocks when blocking ordinary Flash.

The real problem is not that Silverlight is being blocked, I’m sure advertisers will create annoying and distracting ads with Silverlight at some point in the future, it’s that the option to play the control isn’t present like it is with Flash animations. When Flashblock blocks a Flash application, it’s replaced with a white rectangle and a play button. When the play button is pressed, the Flash is enabled. When Flashblock blocks a Silverlight app, your browser thinks the Silverlight plugin is not installed and displays the image requesting you to install it.

There are several ways around the problem. You can simply disable Flashblock when you want to see a Silverlight application, but that requires a restart of Firefox. The other option is to white-list sites where Silverlight is present. White-listing, however, has the side effect of also allowing any Flash on that site to be displayed.

I’m definitely not suggesting abandoning Flashblock, I still use it every day, but if you ever run across a site that uses Silverlight and you can’t figure out why it’s not working, remember your addons might be to blame.