Проверьте работают ли java скрипты в этих броузерах.

Может быть, это?

Trouble with IE7 and Nice Menus
Check the DOCTYPE you're using in your custom template if you're having trouble with Nice Menus in IE7. If IE7 thinks it needs to go into a backwards-compatible "quirks mode" for web pages written to an older HTML standard, a workaround script meant to detect IE6 might not notice. That seems to be the case with 6.x-1.3 where the child menu simply was not coming out at all in IE7 while it worked in Safari and Firefox.

Here's the old DOCTYPE, often used out of habit:

Switching to this one solve the problem:

Nice Menus FAQ: http://drupal.org/node/195157