added a third state to make the “un-highlight” versions (and also the un-ignore button) more immediately obvious

Yes, this seems good.

The highlight buttons also make some sense to me.

The X does seem Ok to me, but I just want to show the sort of mute button I was referring to previously: https://www.google.com/search?q=mute+button&biw=1280&bih=739&tbm=isch&source=lnms

By the way, is there some reason I'm not getting e-mails for this thread?

I can see how button design might be the less interesting part of this problem.

>add a sixth button, for “go to highlighted comments,

Hmm, not sure I'm a fan of that yet. Maybe that's because I continue to use the OT arrows you implemented. I guess any given site could implement either this type of arrows to go among highlighted comments or the other style arrows to visit new comments. Given that there is already highlighting that seems to work well, I don't know that it's needed. But I guess neither does that mean you can't offer it if someone wants to implement it

Unordered thoughts:
1. This seems like a useful thing.
2. Does an "X" icon really make sense? It kind of makes sense perhaps for ignore, but I don't think it makes sense for highlighting the commenter or the comment, even if the X is a different color. I would probably use an audio "mute" button instead of the X for ignore. For highlight comment, I might choose a star. For highlighting the comment, I might use something like this: https://dribbble.com/shots/1462625-Follow-Button
3. It seems to me like the tool tip takes a while to come up. Is there a delay built in, or is that my browser?
4. Looking at the collection of buttons on the right hand side now, it seems to me like their organization needs to be reconsidered. Currently, there are five. It seems to me like the top row should be commenter-level functions while the second row should be comment-level functions. So, the top row should have (1) ignoring the commenter, (2) highlighting the commenter, (4) seeing the commenter's archive. The second row should have highlighting the comment. The remaining icon for seeing the commenter's comments on the the current thread could perhaps go in either one as prescribed by aesthetics.

Of course, perhaps this is difficult. From a plugin perspective, it might be nice to have the icons related to a certain plugin all grouped together.

(Just to clarify, this seems awesome. The above doesn't subtract anything from that.)