Firefox (like most apps), has many pixels of "wasted" space. Most people will likely prefer the space so it doesn't look so crowded. But my preference would be to definitely go closer than the way it is now.

For example, the top bar of the Firefox window has the menu tabs "File Edit View History Bookmarks Tools Help". There are about 3 or 4 pixels of space above and below these letters, making up that bar. The next one down (holds the URL box and the search box) has similar spacing.

Below that we have "Most Visited / Getting Started / Latest Headlines / ${USERNAME}'s links". I have in the past successfully eliminate these words entirely. But the bar never went away entirely and still occupied about 7 or 8 pixels vertically. And fourth is the tabs bar. I'll keep that, bit even this I want to squeeze tighter.

It's probably a text rendering thing, giving a generous default amount of space around letters. I don't want to change source code, though. I'd like to be able to get this compacted by some configuration. Would this be something that could be done within Firefox's own CSS files, or do those just make the defaults for web sites (which I also want to do compacting on, eventually).

If anyone has already done this, that would be great (if shared). Else, any hints, suggestions, or directions to go?

You can check Chromebug extension (it work with Firebug). It is advanced tool, but you can see live changes. You can also change files which define user interface view: ~/.mozilla/firefox/some_id.default/chrome/userChrome.css and your theme files (somewhere in extension directory). Of course you must known CSS language, but what you want is simple.