Users affected by it, Mozilla, any other company that comes to support Mozilla, watchdog groups like the EFF...

It can also be brought by attorneys general and governmental regulators, the FCC and FTC might have a bit to say about it...

Antitrust suits aren't civil cases, I don't think, so "having standing" is a bit different

I'm not a lawyer though so I could be way off base, but the antitrust cases I've been aware of I don't think they were brought by companies but by government agencies