Lightbulbs
 

The things in the l/h column.

On this computer, sometimes they're there, sometimes they're not.

Why is this?

Tells you if there are new posts since your last visit. Yellow means new posts, white means no new posts. Will only show up if you have a cookie from a previous session stored. If you delete your cookie file and log back on the lightbulbs will not be there as it assumes you have not visited before.