URL: https://www.pinterest.com/

Browser/Version: Firefox for Echo Show v1.1; Firefox for Fire TV v2.2; Firefox Focus v7.0.12; standalone WebView app

Operating System: Fire OS ?; Android 8

What seems to be the trouble?(Required)

• Desktop site instead of mobile site
• Mobile site is not usable
• Video doesn't play
• Layout is messed up
• Text is not visible
• Something else (Add details below)

Steps to Reproduce

1. Navigate to: pinterest.com
2. Click "Sign in with Google" (note: on browsers with "tracking protection" or "turbo mode", it must be disabled)

Expected Behavior:
Pinterest login flow begins. Upon completion, user is signed in and can use site

Actual Behavior:
White screen where user cannot take any action.

You can also use this standalone Android WebView app to reproduce.

The sign in flow works correctly in Chrome for Android and Firefox for Android: they open a second tab to start the login and return to the original tab, where the user is signed in, upon completion. Desktop Firefox also opens a second window to handle the login flow. I also added multi-window support to my standalone Android WebView app and it is able to sign in correctly. I'm wondering: is multi-window support required to sign in with Google or does something need to be changed on Pinterest's side?

The reason I ask is because other sites don't require multi-window support to sign in via Google. For example, nytimes.com, IMDB, and imgur all sign in successfully (note: I tried these with the Firefox for Echo Show code base on the emulator).

Screenshot

Firefox for Echo Show hits the white screen.