What I didn't notice in my last post, but should have, was Console² (Mozilla 1.5 or later only), which did just what I was looking for, and more, in about the style I had been hoping for. It plays nice with Console Filter too, as you can see in the screen shots below:
(I suppose we've seen those before, yes.)
Ooo! Now we're talking!
Yes! No more scrolling to death to find the most recent entries, either. Thanks!
I wonder when this extension will enter Mozilla proper, rather than being a sideshow we have to dig up for ourselves. Well, I'm happy about having found it, and thought I ought to spread the word as best I can.