Resolve "Firefox auto-fills your remembered username in the search box if you open the user settings page"

Closes #1090 (closed), supersedes !1106 (closed)

@interfect, I found out an easier solution, which was setting the "search" type on the search field ;) (via https://gist.github.com/runspired/b9fdf1fa74fc9fb4554418dea35718fe#gistcomment-2725790)