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.htacess is not browser-dependent, since it's executed soley on the server.
IE does it's users a "favor" by displaying a "Friendly" error page if the site's error page is short. Try making your custom error page longer and see if that helps.
Also, this may depend upon the "Show friendly HTTP error messages" setting in Internet Options-Advanced.
If you make your custom error page long enough, I believe it will override the setting.
Thanks, all-knowing M$ - We are not worthy...