The problem
In IE my search box was working fine I was able to enter search queries without any problems.
When I tried to search in Firefox (and other browsers) I wasn’t abel to access the search box.
The solution
Recently I added google analytics to the bottom of the masterpage (this is a small javascript).
The following two lines have to be at the bottom of the master page. If other javascripts or anything else follows these two lines then the javascripts used by the search box aren’t generated.
<asp:ContentPlaceHolder ID=”PlaceHolderUtilityContent” runat=”server” />
<SharePoint:WarnOnUnsupportedBrowsers runat=”server” />