If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.
Just tested this on a clean FF profile, so it's almost certainly something on your end. Did you successfully install the script? You should've gotten an image which looks something like this, and if you go to Greasemonkey's menu while on a LW thread, you should be able to see it in the list of scripts run for that page. Also, note that you have to refresh/load a new page for it to show up after installation.
Oh, and it only works for new comments, not new posts. It should look something like this, and similarly for replies.
ETA: helpful debugging info: if you can, let me know what page it's not working on, and let me know if there's any errors in the developer console (shift-control-K or command-option-K for windows and Mac respectively).
I had interpreted “Save this file as” in an embarrassingly wrong way. It works now!
(Maybe editing the comment should automatically uncheck the box, otherwise I can hit “Reply”, check the box straight away, then start typing my comment.)