"Please enter a valid email" message is not displayed if trying to submit an invalid email by pressing the "Enter" key

[Softvision-MariusComan]

[Affected versions]:

• Firefox Release 60.0.2
• Firefox Beta 61.0b14

[Affected Platforms]:

• All Windows

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "Firefox Monitor" add-on (0.01 custom built on 20.06.2018) installed.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Navigate to the "https://fx-breach-alerts.herokuapp.com/" page.
3. Enter an invalid email in the input field.
4. Press the "Enter" key and observe the behavior.

[Expected result]:

• The "Please enter a valid email" message is displayed.

[Actual result]:

• Nothing happens.

[Notes]:

• This issue is not reproducible if clicking the search button from the right side of the email field.
• Attached a screen recording of the issue:

[nhnt11]

This is not related to the add-on, could you please file this in https://github.com/mozilla/blurts-server/issues

[pdehaan]

Moved to https://github.com/mozilla/blurts-server/issues/213