Sign in: The user's selection doesn't work by "Enter" (FF)

Steps to reproduce

1. Sign and logout (so, username will be in browser suggestion list)
2. Click "Sign in", start typing your username.
3. Select username via keybord and press "Enter".

Actual Result: The cursor moved to password field. Username field is not filled. Username can be selected via left\right arrow key.
Expect result: Username can be selected via "Enter" key