I'm confused. That's what I said is the default, and you said it isn't. Now you start by saying "no" to me, and confirming what I had said. Sorry if I've missed something though, I'm really tired right now.
I said "no" to "have it set to not install before prompting you" because I use the default settings. Now I'm actually also a bit confused whether you meant "it installs in background [while browsing]" or "it installs in background [while Firefox is not running, using the background update server]"
With the default "when Firefox is not running" setting, the updates should _download_ in the background and then inform you that it will be installed next time you restart.
The freezing and forced restart as reported by pontifier does not sound like default behaviour, it sounds like "when Firefox is not running" is unchecked.