I faced an issue when trying to edit content in Joomla using the WYSIWYG editor.  In Firefox the popup just shows a blank page and in IE you get a 6001 access denied error.  This is probably due to a configuration file on your web server.  Check the file called configuration.php in the root of your Joomla instalation.  It contains a variable $mosConfig_live_site.  Make sure you have the www after the http:// otherwise you will face the problems that I mentioned above.  

Just modify the file by adding the www and save it.  Once you are done, login and try again and the issue should be resolved. 

Incorrect value
$mosConfig_live_site = 'http://marionweb.com';

Correct value
$mosConfig_live_site = 'http://www.marionweb.com';

Hopefully this will save you hours of searching for a solution on your own.