Can't login in admin panel through firefox and chrome (Opera is fine)

Question

Suddenly I can't no longer access the admin panel from chrome and firefox with magento 1.9.2.2, however chrome is fine.

Settings(Magento url: www[dot]farmaciacaloini[dot]com[slash]shop):

web/session/use_http_via                0
web/session/use_http_x_forwarded_for    0
web/session/use_http_user_agent         1
admin/security/use_form_key             1

admin/security/session_cookie_lifetime  86400
web/default/cms_no_cookies              enable-cookies
web/cookie/cookie_lifetime              86400
web/cookie/cookie_path                  [slash]shop
web/cookie/cookie_domain                [dot]farmaciacaloini[dot]com
web/cookie/cookie_httponly              0
web/cookie/cookie_restriction           1

Observation:

• If the credentials are wrong then the message shows up
• If the credentials are corect the page quickly refreshes and deletes all the cookies
• I have cleaned session and cache folders and memcache and mod_pagespeed cache
• The hidden input form_key is present
• It has worked since some days ago, I haven't done any changes to cookies setting, maybe I have changed some values to the security option, but I'm not sure

Answer 1

The problem was mod_pagespeed.
I have tried to disable the core and progressively enable filters to find out the problem, without success, so I had to completely disable it.