I hope not. Tabs is a feature that I use a lot. Tree tabs seems to take a lot of screen space and I have little to spare on my laptop. MS has conditioned me to become very irritated when my personal feature set is improved or deep sized. The first thing I do when I get a new machine is spend a few hours getting the classic mode to work. I hope FF isnt going going down the same road.
I agree that this implementation of tabs is somewhat ugly, and not necessarily as functional. Tree style tabs don't have to take up as much room, however. The tabs side bar from Tree Style Tabs (an extension) can be resized. Also, trees can be set to not indent if the user chooses. It's nice, because if you need to see the titles of your tabs (I usually prefer to), you can move it over. If not, you can give yourself move space. Furthermore, tabs to the side allows nearly 50 tabs to be visible at once, rather than 20. However, I agree that this shouldn't be the default.
What would be nice is an option to group/tag tabs like bookmarks, and collapse, expand, and explore tabs that way. I know at least one cranky person will say "Well I never have more than 3 tabs open!" Well, cranky strawman, when I read my daily rss feeds, I often wind up with 50. An option (not necessarily a default) to browse tabs by category could be useful.
I agree completely on the Ubiquity bar. If this is implemented, I really think it could bring a major shift in browsers, and I hope Firefox's competitors will also implement it.
OTOH the Ubiquity bar looks to be useful.