User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:30.0) Gecko/20100101 Firefox/30.0 (Beta/Release) Build ID: 20140605174243 Steps to reproduce: Set privacy setting to "Keep [cookies] until": Always ask. Visit a page that attempts localstorage operations. Actual results: The page tests if window.localStorage exists. An exception is thrown. Exception... "The operation is insecure." code: "18" nsresult: "0x80530012 (SecurityError)" location: "<unknown>" A large amount of websites "hang" because of this as javascript execution stops there. For example sqwiggle.com and bitcoinwisdom.com Expected results: The check if(!window.localStorage) should allow scripts to execute without local storage support.

Confirming this also happens on linux. Tested on 31.0a2 for Ubuntu 1.0

Update: once the cookie policy has been set to any of Allow or Allow for session, the exception is not thrown. However Block still throws the exception. And if the policy for this website is unknown the default is to throw the exception.

A very similar issues occurs with accounts.firefox.com, which is now bug 1094655 on the Firefox side (and bug 1094652 for the server side).