There's this loop in test_object.html: for (let from_state of testdat['from_states']) { for (let to_state of testdat['to_states']) { for (let mode of testdat['test_modes']) { for (let type of testdat['tag_types']) { runSoon(function () { let obj = document.createElement(type); obj.width = 1; obj.height = 1; let state = {}; state.noChannel = true; state.tagName = type; // Part of the test checks whether a plugin spawned or not, // but touching the object prototype will attempt to // synchronously spawn a plugin! We use this terrible hack to // get a privileged getter for the attributes we want to touch // prior to applying any attributes. // TODO when embed goes away we wont need to check for // QueryInterface any longer. var lookup_on = obj.QueryInterface ? obj.QueryInterface(OBJLC): obj; state.getDisplayedType = SpecialPowers.do_lookupGetter(lookup_on, 'displayedType'); state.getHasRunningPlugin = SpecialPowers.do_lookupGetter(lookup_on, 'hasRunningPlugin'); state.getActualType = SpecialPowers.do_lookupGetter(lookup_on, 'actualType'); test_modes[mode](obj, from_state, to_state, state); }); } } } } Until bug 1263355, for-let-of scoping has been broken: instead of a fresh let being made per iteration, only one is made for the whole loop. So, that loop doesn't correctly loop over all configuration combinations. Bug 1263355 fixes the scoping but breaks this test, because as far as I can tell, it's not running configurations that never ran before and never passed.

Comment 0 correction: s/it's not running/it's *now* running

Resolving as wont fix, plugin support deprecated in Firefox 85.