User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0 Build ID: 20170130115108 Steps to reproduce: I am trying to port one of my addons to WebExtension but have not been able to progress far due to security and privilege settings and/or lack of suitable API in WebExtension. I need to convert a local image that is displayed by the browser (ie 'file:///C:/....png') to DataURI in order to POST via XHR. Both 'new File()' and 'canvas' run into security problems. eg: > "SecurityError: The operation is insecure." > "Security Error: Content at moz-extension://6715edd4-5ec9-42c7-adea-daa9e297c57c/_generated_background_page.html may not load or link to file:///C:/Users/.....png." I understand that these Web APIs are designed with the added security since they are meant to be used in content script. However, they do not have any extra privileges when run from background script. At the moment there isn't any other filesystem API (eg chrome.fileSystem) (ref: bug 1246236)