Fantastic! Simple and straightforward. And unobtrusive, to boot. The easiest and quickest way to add proxies and rotate among them.
Only one suggestion for improvement: please make the Current Proxy field blank if none of the proxies currently at work. Once one turns a proxy off, the field should become empty, without showing the top one on the list. There's always a risk of accidentally removing a valuable proxy if it happens to be this top one.
Otherwise, just excellent! (Sometimes it seems the proxy lingers for some time after turning it off. Very weird feeling, though.)

Updated:
Your new design looks splendid. Thank you for listening to us, users. It's quite a commendable promptness on your side. But now Proxy Manager takes over proxy settings (there goes unobtrusiveness)! There is no way to switch it off other than by disabling the extension. When one selects a proxy and turns it on, traffic goes through it. But once this proxy is turned off, all the traffic goes through the top proxy on the list. Closing Firefox won't help. This is reflected in the Firefox proxy settings. It's not just lingering now, it's something else altogether.