Button disappears from Navigation bar

I used Fireworks 8 to build a navigation bar and imported to Dreamweaver 8. Each button points to a different frameset. All the links work fine except when, after clicking the first button and clicking the browser back button, the first button then shows a white rectangle. Clicking on the white rectangle still links to the right page but the button is gone. This is true using Safari and Firefox but the problem doesn't show in IE7. All the other buttons work fine in all browsers. What the hey???