Thank you for the replies.
So it looks pretty frustrating for me, since I was trying to develop an addon, which needs to modify the tab bar.
I will try userContent.css first, but I don’t know if it does what I want. (I want to do something like TreeStyleTab, but not in a sidebar)
I have seen that there will be an API for creating a toolbar, I hope it won’t be too troublesome to make a fake tab bar.