Since about a week in two windows and linux I'm having this problem

Case of use:

I'm browsing google reader and when I try to open a feed it opens a new
empty tab and then it open a new window with the required url.
Before those changes, the url opened in a tab.
Settings has not been changed in any of the Ff installation

Current related settings:


If I set to 0, the same
behaviour applies. Opens in a new window with an empty tab.

I'm doing something wrong?

Help is appreciated