FWIW using the Add-Ons tab's search for "tree style tabs" gives 9 results, none of which include this add-on.

But when I click "269 more results", then Tree Style Tab is the first result on that addons.mozilla.org page.

(This isn't the first time I've found add-on search to be a bit wonky. Maybe "everyone" knows to ignore the first results (?), but I don't search for and install add-ons frequently enough to have learned that, quite yet.)

Except Tree Style Tabs makes Firefox 29 look very bad. For some reason, it causes the top part of the chrome to be two shades of gray. The area with the title, and minimize maximize buttons, its one shade, and then the area with the address bar and buttons is another shade of gray. With a harsh line where the two areas meet and are a different color.

Do I have to install an add on to get narrow tabs?

This is like the 3rd or 4th time that minimum tab width settings have been changed. I understand the motivation to change the experience for the typical user. I don't understand the constant churn in what is customizable. If I customized the location of the reload/stop button last time it was moved, is the thought really that I won't want to this time?

I guess there is some argument about the amount of code that needs to be maintained, but if things go the way they go, there will be incremental changes for a year or 2 now, and then a big jarring shift to the new thing that is more fashionable.