- Type about:config in Firefox address bar.
- If it displays a warning, confirm you know what you're doing.
- Find this property (you can use the filter): browser.display.focus_ring_width
- Set its value to 0 (zero).
Oh, how annoying that dotted line around linked elements in Firefox is! There are many articles describing how to remove it for your website. I don't like the idea - it should be up to the user to decide. So I, as a user, decided to remove them from my browser. How?
This time this helped with my VirtualBox problems (Ubuntu host, Windows guest).
sudo dkms install virtualbox/4.1.2 sudo modprobe vboxdrv