In the upper left corner there is a box that reads "THREAD OPTIONS" next to "Tools" the drop down menu should take you from there.
I have that option when looking at a thread. But that only works for comments in
that thread. No such option when I am looking at the forum and would like to "Start" a new thread. When viewing the forum the only button on the upper left side is to "Follow" the forum.
edit: And obviously others do have an option to start a thread as this one, and others, have been started since the upgrade. So once again, I am not just like all the others. Of course that may also be reflected in my having a Triumph and not a top selling brand.