First, I dislike that I cannot quickly count how many things there are. Borders also make me feel more confident in locating a button.
Second, by detaching the borders from the main window it disconnects the analogy of a "tab" as a, well, tab. Now a button takes you to a new window, but I dislike that it breaks a long-standing analogy that is easy to grasp.
More importantly, it's really hard to see the upside. That's usually why people complain about UI changes in Firefox or anywhere else -- if you can't elucidate what the upside is, all anyone can see is downsides. And mild lack of familiarity is enough downside to outweigh an inexplicable change with no upside obvious.