Backed out 3 changesets (bug 1309699) because this wasn't supposed to land on trunk a=backout

Backed out changeset b10e48ea5d20 (bug 1309699)
Backed out changeset 829c16e72fb7 (bug 1309699)
Backed out changeset 559fa765dbf1 (bug 1309699)

toolkit/components/thumbnails/test/browser_thumbnails_bg_no_cookies_stored.js

@@ -4,10 +4,6 @@
 // check that if a page captured in the background attempts to set a cookie,
 // that cookie is not saved for subsequent requests.
 function* runTests() {
-  yield SpecialPowers.pushPrefEnv({
-    set: [["privacy.usercontext.about_newtab_segregation.enabled", true]]
-  });
-
   let url = bgTestPageURL({
     setRedCookie: true,
     iframe: bgTestPageURL({ setRedCookie: true}),