Object on other grounds: the brief is filed too late, it raises waived issues, the brief does not conform to the rules, the cover sheet is the wrong color. Whatever.
But please, do not object on the basis that “amicus curiae” means “friend of the court,” and the filer is really a friend of the other party.
Especially when amicus practice is well-accepted. They even write law review articles about it.
And if you still are intent on objecting on that basis, please don’t cite a 1903 case as your sole support. You’ll just look desperate.
Objection to Motion for Leave to File Amicus Curiae Brief, Corboy v Louie, No. 11-336