It's seems to be like Retrofit recognizes colon symbol in relative as schema like https in the url path. So i need directly put full url the API interface by passing it through arguments using @Url annotation:

@POST()
    Call<AuthResponse> signIn(@Url String url, @Body UserBody user, @Header("Content-Type") String contentType);

OR

According to the github issue i should add to the relative path ./ as first symbols. So it'll be looks like this String signIn = "./accounts:signInWithPassword?key="+FirebaseConfig.API_KEY;