From FCKeditor Docs

ImageBrowserURL

Defines the URL to load when clicking the "Browse Server" button in the image dialog. It can point to the default File Browser

FCKConfig.ImageBrowserURL = FCKConfig.BasePath 
+'filemanager/browser/default/browser.html?Connector=../../connectors/' + _FileBrowserLanguage + '/connector.' + _FileBrowserExtension ;

or for any custom implementation:

FCKConfig.ImageBrowserURL = '/myfilebrowserpath/browser.php' ;