You could change the nav.li from float: left to display: inline-block , after that you might have to adjust your main nav padding and apply related changes to the dropdown elements.
Hope this helps,
Thank you very much Nicky, could you be more precise? Exactly which files and lines of code I have to change from my App or Skin folders?
Fortunally I have no dropdown elements, only main nav.
if you’re working on the default template (as per your screenshot) you will need to apply the mentioned changes in the related css file (skin/frontend/default/default/css/styles.css) - once the operation has been performed you might clear your cache and it should work.