How do I prevent the Flash Player installer from also installing McAfee, which it seems to do automatically with no cancel option, and which I do not want and will not accept?
Are you going to get.adobe.com/flashplayer to download the installer? Are you using an ad-blocker? If so, it will block the ad and prevent you from un-checking the offer.
FWIW, I also see occasional complaints about this from Firefox users who have updated from the Firefox plug-in manager, instead of coming to Adobe directly. In testing current Firefox versions, they've taken me to the the appropriate download page, which works correctly. If it's an old Firefox version, that may be what's going on.
Please go directly to the Adobe download site to ensure the intended experience (and as Maria pointed out, disable any ad-blockers or anti-tracking plugins that may prevent the page from working correctly): http://get.adobe.com/flashplayer/