Rated 4 out of 5 stars

I was able to get this working with 3.6 by performing these changes.
{your profile}/extensions/pip@mozilla.com/install.rdf

Change max version to 3.6

{your profile}/extensions/pip@mozilla.com/pip/content/pip.js

Replace starting at line 114

if(elem instanceof Components.interfaces.nsIDOMHTMLEmbedElement) {
var embedBox = elem.getBoundingClientRect();

com.taboca.pip.gPop_x = embedBox.left;
com.taboca.pip.gPop_y = embedBox.top;
com.taboca.pip.gPop_width = embedBox.width;
com.taboca.pip.gPop_height = embedBox.height;