Firebase google auth not authorised on ios, but is on android

My app uses facebook & google sign in with firebase signInWithRedirect.

It works perfectly on an android device, but not on iOS.
On iOS I’m getting

Auth/operation not supported in this environment
This operation is not supported in the environment this application is running on. \"location.protocol\" must be http, https or chrome-extension and web storage must be enabled.

It’s important to note that login password authentication through firebase works on iOS.

I’ve tried several things, such as adding a GoogleService plist file so that my iOS app would be registered at Firebase. But it doesn’t seem to be picked up in XCode.
Not sure how this works, because the firebase javascript config object has nothing to do with the platform… maybe I’m missing something here…

I’ve tried to use in-app-browser too, but no luck either.

This works on android

<access origin="*" />
    <allow-navigation href="*" />
    <allow-intent href="*" />

Can somebody think of something I’m missing at all?

Thanks a lot