i've been keeping a cursort interest in the "Australis" interface that is being developed as the new interface for firefox. It looks both interesting and different.
Recently I have not been too much a fan of different or change, so I was a little wary over this new interface.
While playing around on Fedora 20 (I like where Gnome 3 is going...), I installed the test repo for the gtk3 port of firefox and I must say that the new interface was quite pleasant. I always thought that the rounded corners for the new tab design were too round and that background tabs would look too hidden, but these were not a concern in actual usage.
I am not sure when the new interface will hit firefox (I think its been pushed back to around March/April?), but when it does, I think it will turn out to be quite good.