I have FF 1.5.0.1, and have had this problem since the upgrade to 1.5. When i switch from one website to another using the url's in the address bar, the current page refreshes and doesn't load the new page. About half the time it works but the other half it resorts back to refreshing the same page.
I''ve never encountered any issues like that whatsoever. Like Ronin said, try to uninstall, then download its latest version and reinstall back on your system. Hope that'll resolve the issue. Since the first day I switched to FireFox last May of 2005, I'd never seen any pop-ups lately.