Firefox shows 404 Error when accessing pages

To be honest, I have no clue where the problem is: Wordpress or maybe Atahualpa.

My installation:
Wordpress 2.7.1
Atahualpa Theme 3.3.2
Permalinks are set to default.
Wordpress installation in subdirectory
index.php in root (path to wordpress adjusted)

Error Description:
When I click a pagelink (like ?page_id=5), firefox will not open the page, instead presents my custom 404 Error Page.
When I open the same link in a new tab, the page is displayed correctly.
It only happens using Firefox (tested with version 3.0.7/3.0.10). Not matter whether Mac or Windows
Safari and Internet Explorer do not show this behavior. They work fine.

Can anyone help?