The internal WebBrowserPersist code does not use correct origin context for a resource being saved

The internal WebBrowserPersist code does not use correct origin context for a resource being saved. This manifests when sub-resources are loaded as part of "Save Page As..." functionality. For example, a malicious page could recover a visitor's Windows username and NTLM hash by including resources otherwise unreachable to the malicious page, if they can convince the visitor to save the complete web page. Similarly, SameSite cookies are sent on cross-origin requests when the "Save Page As..." menu item is selected to save a page, which can result in saving the wrong version of resources based on those cookies. This vulnerability affects Firefox < 63.