A great addon that saves a ton of time if you browse Reddit frequently or use the weekly/monthly feed.

It would be nice to have a 5-10 second delay before hiding posts that went off-screen.
Or, for example, to show the last 5 hidden posts (before refreshing the page).
The problem is that when you scroll quickly through the feed, the off-screen posts are already hidden and you can't scroll back up if there was something interesting there.

Also, the 750 post limit is quite low (in fact, it's even lower due to the way database cleanup works).
Perhaps it's worth using a sync store for recent posts and persistent storage for recent + older posts.
If a post only uses 10 bytes, it won't be a problem to increase the limit to 10-50k using persistent storage.

P.S. I suspect that some posts may be hidden by mistake because the browser remembers the last scroll position and sometimes displays the feed not from the start. I'm still testing this.