Firefox didn't "trick" you, Adobe did.

http://get.adobe.com/flashplayer/ has a checkbox next to "Free McAfee® Security Scan Plus (optional)". You would have to contact Adobe about your objection to this.

This McAfee product shouldn't disable other AV applications, and can be uninstalled from an item in its Programs folder in the Start Menu: See the bottom of http://us.mcafee.com/en-us/affiliates/adobe/landingpages/np681.asp?cid=72563

"Firefox didn't "trick" you, Adobe did."

Yes, but Firefox is driving the getaway car. Updating Firefox requests you to update Adobe. Updating Adobe directly (i.e. from the Firefox warning screen, not starting at the adobe website) forces a download of McAfee. You can then un-install McAfee but at the next Firefox update, you're asked to update Adobe again. At least, that's what seems to be happening to me. I'd prefer that Firefox's check was able to determine that Adobe is current and ignore McAfee's status completely. Without that more refined check, Firefox is an accomplice, albeit unwittingly.

I updated Adobe Flash today and there was definitely no place to uncheck installing McAfee. There was a box to uncheck installing Chrome.

I read in an Adobe Forum that "For some reason Firefox uses the DLM. I recommended not using it and to Manually Install Flash Player into FF or IE."

It was easy enough to uninstall McAfee, but I am appalled at what happened.