It's 1.1 mb, which works out to less than five seconds for most people. I suppose they do leave it out to keep FF more lean, but that doesn't mean it Mozilla can't have an official web development add on.
1.1mb multiplied by the number of downloads of Firefox divided by the proportion of people that will use them? I don't think that that sounds like very good value...
Not to mention that any developer worth his or her salt already knows to install Firebug, and everyone else doesn't need to.
Chrome style incremental updates would help here. Firefox makes people download the entire browser package each time there's an update, while Chrome downloads a patch - a saving of >90% size-wise.