If they ran with user privilege, they could still stomp on each other.

The OSX preferred app install process (used by FF) is simply to download the disk image, and move your app to the /Applications folder.

The Mac App Store likely also prevents apps from clobbering other apps' folders while installing.

I try to avoid software the requires an installer if at all possible.

There should be sandboxing / requesting priveleges like Android has it.