Thank you for this script, it does the trick !
The result is just what it is expected to be. Simple and straight functionality add-on like I love them.
It adds much more readability with such a tiny enhancement and saves so much time not looking for the last read tweet since new update.

Just keep it "KISS" like it is.
(cf. https://en.wikipedia.org/wiki/KISS_principle for "KISS" definition)

Yes! Twitter is so much better when you can read up, instead of down. This is the script I installed Greasemonkey for.

Minor suggestion: the bottom border on the last unread tweet seems a bit heavy for Twitter's look, and the 0-pixel side borders make it seem to bulge out beyond where it should. I tweaked the style as follows, and it seems to work nicely (I also gave it a hint of blue):

{ border-bottom: 1px solid #808088; }

I don't really know anything about CSS, including what !important means. I left it off and I didn't notice any difference, so I figure it's probably...not important.

(I'll be here all week. Try the veal.)