No, Mackeeper does not come with Firefox.

Only download Firefox from https://firefox.com

Mozilla installers only install Mozilla programs. Where did you get your from?

Legal said

When I downloaded I got MacKeeper that I let do a sweep. Bad idea.

It is Firefox and not Foxfire.

Firefox from *.mozilla.org like say www.mozilla.org/firefox/all/ for Windows, Linux and Mac does not come bundled with any software. It would be a hot topic here and elsewhere if it did.

From this thread /questions/1129429 a Mac user says a Ad him instructed to download a new MediaPlayer, which just seems to download Mackeeper. Maybe you already had it on system already by some means and only just noticed it sometime after downloading Firefox.