[nesyt]

Is that a built-in feature or do you use an extension?

[berberous]

I use the "Tree Style Tab" extension, available here: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

If you haven't used it before, and if you're prone to having 10+ tabs open at a time, I highly recommend it. Although I warn you that you may never wish to use Safari or Chrome again...

Chrome used to have a similar feature built-in but they removed it ages ago.