I installed and ran the free version of Simple Popup and it woorked fine until I closed the window. Then I didn't work until I cleared the cache and cookies.
I paid for and downloaded and installed the premium version and it never worked at all - no popup appears no matter what I tried.
Still waiting to hear from support.
Also note that the current free version is supposed to be 1.6.8 but when you download it you get 1.6.4.