"Invalid OAuth parameter: prompt" error when trying to log in with FxA with Mozilla email

[rleitan]

[Environment]:
Windows 10 - Firefox Nightly latest

Precondition:
Discouse staging account with LDAP (e.g. [email protected])
FxA account with the same email

STR:
1.Login with LDAP on Discourse staging
2.Log out
3.Click Log in button from Discourse staging
4.Select “Continue with Firefox” button from NLX screen
5.Enter valid credentials-> login error (expected)
6. Click “Go back” button then select “Continue with Firefox” button from NLX screen

[Expected result]:
"Staff LDAP account holders are required to use their LDAP account.." error should be displayed

[Actual result]:
Invalid OAuth parameter: prompt