Investigate whether the Symbol data type can store global identifiers accessible to content
"To create symbols available across files and in a global scope-like environment, use the methods Symbol.for() and Symbol.keyFor() to set and retrieve symbols from the global symbol registry." (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol)
If we need to disable this feature https://bugzilla.mozilla.org/show_bug.cgi?id=1066322 shows a way Mozilla used.