This only seems to be a problem now with firefox 3.6 When you click anywhere on the page which is one big embedded silverlight object the little broken outline appears, the same as when you say click on a link. This is causing the page to go one pixel over horizontally and vertically which is causing scroll bars to appear.
Any idea?
Try setting outline-width:0 for your object tag either in your CSS file or directly inline on the object tag for the plugin:
<object data="data:application/x-silverlight-2," type="application/x-silverlight-2" style="outline-width:0">
Seems to be the side effect of some kind of focus ring usability bug that was fixed for 3.6, which is causing this with Silverlight, Flash, Java Applets, etc.
http://www.blog.lessrain.com/firefox-36-draws-one-pixel-border/