Firefox only shows an empty space instead of the opt-out iframe when loading it from another domain, using piwik 2.12.1 and firefox 37.0.1. Probably a problem with the X-Frame-Options setting, I checked headers with
curl -I and it seems like X-Frame-Options is empty for the iframe url, while being set to sameorigin for other urls. Just tried with oxfam.de, same result:
$ curl -I "https://www.oxfam.de/piwik/index.php?module=CoreAdminHome&action=optOut&language=de" Date: Wed, 15 Apr 2015 21:47:00 GMT Server: Apache Cache-Control: no-storPragma: Expires:e, must-revalidate X-Frame-Options: [..] Vary: Accept-Encoding,User-Agent Content-Type: text/html; charset=utf-8 $ curl -I "https://www.oxfam.de/piwik/index.php" HTTP/1.1 200 OK Date: Wed, 15 Apr 2015 21:48:46 GMT Server: Apache Cache-Control: no-store, must-revalidate X-Frame-Options: sameorigin [..] Vary: Accept-Encoding,User-Agent Content-Type: text/html; charset=utf-8
Found the relevant code, it is intentional to send an empty x-frame-options: https://github.com/piwik/piwik/blob/3838/core/View.php#L244
Not sure whether the empty X-Frame-Options is the actual reason for firefox not displaying the iframe, though.
Found the solution: It's not Firefox, it's the adblocker filter lists I use there, which also contain generic matches for urls starting with piwik. TIL.