input format and similar drop downs menus are not clicking under Firefox 3 !!

Many of those drop down links are not clicking, the white arrow becomes up / down arrow, but there's no content beeing showed under the link. It just happens with Firefox. The same situation under IE6 is working fine.

I'm using fresh install with basic modules and the core theme.

I'm also having this javascript error :

Error: doc.body is null
Source File: http://localhost/drupal6/misc/jquery.js?a
Line: 13

thanks!