self.Symbol.for is undefined in a webextension in FF

[kirlat]

We're using a system-observable inside a content script of a webextension (as a dependency of the Apollo client). In the webextension, root is self so it uses a self.Symbol as a Symbol object (https://github.com/benlesh/symbol-observable/blob/master/es/ponyfill.js#L3). Unfortunately, we've discovered that in content scripts of a webestension in Firefox (as of version 83) the self.Symbol.for is undefined, even though self.Symbol is a valid object with the for property available.

As a result, result = Symbol.for('https://github.com/benlesh/symbol-observable') (https://github.com/benlesh/symbol-observable/blob/master/es/ponyfill.js#L13) throws an unhandled error (Symbol.for is undefined) and the content script loading fails.

Even thought this probably is an issue on the Firefox side (it works in Chrome perfectly), I think it might be worth adding a check for Symbol.for before calling it. That would help to avoid the problem described above now and in the future, in various environments.

What do you think? I can create a PR if you don't mind.