The Showcase thing didn't came on by default and CTRL+TAB is not showing the preview, only switches between tabs. I had to enable it in about:config / browser.allTabs.previews (true), browser.ctrlTab.previews (true) and browser.taskbar.previews.enable (true).
The first time I used Tab Groups an icon appeared and now I can't make it dissapear??? (Without removing it from the bar with "Customize") I believe that's the normal behavior.
I had to move the Home Button to the left (with Customize option). It just was too difficult to access.
I have many add-ons like adblockplus, dowloadstatusbar and downloadhelper so since I like not having a statusbar I had to put the buttons next to the Address Bar so now my brand new installation loos awful :/
The New Tab menu (in the Firefox menu) pops and won't let me select my bookmarks, even worse the "New Tab" menu is duplicated inside its own menu.
There are like five ways to open my bookmarks, 1. on the button to the right of the toolbar, 2. on the Firefox menu, 3. clicking on the bookmarks menu inside the firefox menu button (maybe this one not since it's the library), 4. pressing CTRL + B to dock it to the left of the screen, 5. pressing "ALT" and selecting Bookmarks. Why so many ways???
With the History bar it's similar 1. with Firefox menu and then browse to History, 2. "CTRL+H", 3. clicking on the History menu inside the Firefox menu button (maybe not... again), 4. "ALT" and History.
No "Save tabs" when closing FF with multiple tabs open. It saves them by default :/ but it can be changed it back to your Homepage in the Settings menu (General tab).
Where's the zoom button??? I have to press "CTRL +" or "CTRL -". You can add one if you want in customize menu, right clicking the Tool Bar.
Oh and BTW I use Win7, 32bit, with an NVIDIA GeForce 8400M GS video card (with the latest drivers).