Trust me, once you get used to it, going back to the Firefox 4 behavior is just very frustrating.
Arguing about how efficient you can be /after/ you've targeted a microscopic UI element with your mouse cursor seems odd.
I would again ask that you actually go try it rather than assume you know what I'm talking about. Trust me, the way I can target a tab and rapidly close lots of tabs (YES, by using the middle click SPECIFICALLY) does NOT work in Firefox 4. Please try it yourself rather than force me to make a video of it.
Open 8 tabs in Firefox. Target the third tab and without moving your mouse, close tabs 3, 4, 5, 6, 7, 8. It's not possible. The tabs resize and you end up closing tabs 1 and 2. I don't know how you browse the web, but I daily find myself opening numerous tabs and then wanting to close the last several after opening lots of pictures or articles on reddit. (But then again I'm in Linux and scroll on my tabs in Chrome so I move my mouse very little as it is and it suits my browsing habit.)
Also, I browse with browser.tabs.closeButtons set to 3, which gives me a fixed-position close button at the end of the tab bar. (And on my laptop, I close tabs with a gesture that doesn't require me to move the cursor at all.)
Delayed resizing is a neat feature, and one that Firefox should copy, but it is not as critical as you make it out to be.
How on Earth can you possibly say this? Besides it being a matter of opinion, it's highly lauded as good UX (which I believe and have heard numerous other Chrome users swear by), and you can't possibly know how often I use it or how frustrating it is to use Firefox with it's default behavior.