Just perfect! I switched from Chrome recently, where I used a similar, but much inferior extension. This one is among a few reasons why I'm probably not switching back to Chrome anytime soon. Popup window is a real helper. And I wish Firefox itself had such a flexible and easy way of adding search engines as this extension has. One minor drawback - it seems that the extension doesn't cache search engine icons, so they are sometimes redrawn one by one rather slowly.
I could not find a nice solution for this issue. A solution, which will work only for the context menu, is do download on your computer the icons. Then, in the Options page, set the "Icon address" for each row to point to the local file. This address should be like "file:///C:/icons/google.ico".