I use Opera 6.05 as a browser, and I've never had a pop-up at all on Mudcat, nor any other problem.
I don't us Opera 7 because my scrolling mouse doesn't like it (it has self-scrolling functions), IE because it is slower, and Netscape is irritating to me for reasons I haven't analysed. I have them all installed, and sometimes two running at the same time, when I want to transfer something. You could try installing another browser and seeing if that improves things.