RequestPolicy is a Firefox extension that gives you control over cross-site requests. --- Be sure to look at the dev-1.0 branch as that's where all of the interesting work is happening. See also: https://www.requestpolicy.com/1.0.html
When RequestPolicy 1.0 is turned on with default blocking, the new "Sign in to Sync" page will display, but the pressing the "Get Started" button displays a blank page.
It is not enough to click the RequestPolicy "Disable Blocking" after the "Sign in to Sync" page is displayed, "Disable Blocking" must be clicked before selecting "Sign in to Sync" from the new Firefox main menu.
When RequestPolicy 1.0 is turned on with default blocking, the new "Sign in to Sync" page will display, but the pressing the "Get Started" button displays a blank page.
It is not enough to click the RequestPolicy "Disable Blocking" after the "Sign in to Sync" page is displayed, "Disable Blocking" must be clicked before selecting "Sign in to Sync" from the new Firefox main menu.