To reproduce:

1. Open Firefox on Linux (with or without content blocking enabled), no addons.
2. Go to https://my.minecraft.net/en-us/login/
3. Type in any syntactically correct email.
4. Type in anything in password field.
5. Click "Log In"

Expected result:

1. Logged in.

Actual result:

1. Nothing happens.
2. Console gives the error Unhandled promise rejection TypeError: "NetworkError when attempting to fetch resource."

Full console output is in screenshot attached.