StumbleUpon bug!
After much digging into the Wizz RSS code to uncover the cause of the toolbar weirdness, I have finally found it!
The Wizz RSS code is not at fault! I had to dig into the StumbleUpon code to find the real cause, and here it is: -
In stumbleuponOverlay.js you’ll find the following: -
var toolbars = document.getElementsByTagName("toolbar");
var i;
for (i = 0; i < toolbars.length; i++)
{
if (! toolbars[i].hasAttribute("customizable"))
continue;
if (toolbars[i].getAttribute("customizable").toLowerCase() != "true")
continue;
su_is_item_in_toolbox(true, "su_toolbaritem", toolbars[i]);
if (toolbars[i].id == toolbar_position)
su_place_toolbaritem(toolbars[i], position_group);
}
If you understand JavaScript you’ll notice that the code gets all elements with the tag name toolbar. Regardless of whether these elements belong to StumbleUpon or not, the code then proceeds to check each toolbar element for the customizable attribute, and if the customizable attribute has any value apart from true, the code performs all sorts of nasty things to the toolbar in the su_place_toolbaritem() function.
And Wizz RSS’ crime? It sets the customizable=”false” attribute for its toolbar! The customizable attribute is perfectly admissible, and can be used by any toolbar - See Mozilla documentation at https://developer.mozilla.org/en/XUL/toolbar#a-customizable
So, all of you who have been experiencing toolbar weirdness, I suggest you get on StumbleUpon’s case 
