To my surprise I was looking for the "Mark this forum at read" but it changed place and isn't always present.
Not sure if this is of importance, the theme I'm using is "Cobalt", this one matches actually very nice with the Xfce header :-)
Anyhow, this link used to exist at the top of the page when you go inside a forum. Now it is at the bottom, and is not present in case you only list the "New posts".
HTH in case someone else is wondering.