Cookie names are case-sensitive. The RFC does not state that explicitly, but each case-insensitive comparison is stated so explicitly, and there is no such explicit statement regarding the name of the cookie. Chrome and Firefox both treat cookies as case-sensitive and preserve all case variants as distinct cookies.
Test case (PHP):
print_r($_COOKIE);
setcookie('foo', '123');
setcookie('Foo', '456');
Load script twice, observe $_COOKIE dump on second run.