Make Safari custom User Agent change permanent

I need to navigate a site that makes use of javascript that checks for user agent and only permits IE9.0 (don't ask). I am using safari and can set IE 9 in the user agent submenu of the develop menu. However, when the site directs me to another page in its collection, an exception is thrown because the new page sees I am using Safari. Reseting the user agent in the develop menu does not work because the page reloads as a safari user agent. Yes I edited the com.apple.Safari plist to include the following item: Key: CustomUserAgent class: String value: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0) But safari deletes that entry every time it opens a new tab/window and reverts to default! The plist reverts back to 'normal'. Is there a way to make this change permanent?