Error establishing a database connection

I have a problem with my database connection in firefox browser. It says: Error establishing a database connection. In the wp-config.php I set the database name, login and password correct as I made them in my providers interface for making a database. On the site of my provider they say “localhost” is their DB servername. But it still gives me that error message!

What can I check and eventually change? The browser starts at http://www.mydomain.nl (and the home.php is loading, everything looking good) but when clicking on a link in the menu it gives that error (and the url is http://localhost/?page_id=10. Is that correct??

thanks already!