gmail http_user_agent GoogleImageProxy gives incorrect browser details

[HTTP_USER_AGENT] => Mozilla/5.0 (Windows NT 5.1; rv:11.0) Gecko Firefox/11.0 (via ggpht.com GoogleImageProxy)

I am in email marketing. I want to record the browser which opened the email sent by us on gmail. For every Other client we can get this information from HTTP_USER_AGENT but gmail sends above HTTP_USER_AGENT string for all the scenarios i tried eg. Windows/firefox,Windows/Crome,Mac/Firefox,Mac/Crome.

Can anyone suggest a solution to this. I want to get the browser on which email sent by us is opened. Something that can be developed and not use a third part tool.