There might be other reasons, but a big one is that having search and address bar in one box means that normal URLs you type into the address bar will get sent to the search engine for autocomplete by default. Since that's a major privacy violation, they're kept separate.
I've removed the search box myself. If DNS doesn't return anything or if what you type looks like a search query, it does a search anyway, so the only thing I'm missing is search autocomplete (there's an addon if you really want that anyway: https://addons.mozilla.org/en-US/firefox/addon/instantfox/).
The privacy reason honestly seems like a post-hoc justification... you can easily turn off auto search in Chrome. If you were really anal, you could do what IE does and actually _ask_ the user for permission before hitting the search engine. There is really no reason why the search bar has to be kept separate, other than user comfort. And that is not a bad thing: there is no need to start making up excuses for it.
I disagree. When I type something into the search box, I'm explicitly saying "send this to the search provider". I'm more than happy for that box to auto-complete. However when I'm typing into the URL bar, I want different behaviour. That data should remain private.
When I'm saying is that I want both autocompleted search and a private URL bar.
Even if it's turned off you can leak information. Mis-type a local hostname and suddenly your secret URL is public to Google. I've done this more than a few times.
I've removed the search box myself. If DNS doesn't return anything or if what you type looks like a search query, it does a search anyway, so the only thing I'm missing is search autocomplete (there's an addon if you really want that anyway: https://addons.mozilla.org/en-US/firefox/addon/instantfox/).