Windows 7 taskbar previews to be disabled by default in Firefox 3.6
It was already known that jump list support won't be arriving until later on. After a lengthy discussion on Bugzilla, the decision was made to disable Firefox 3.6's other Windows 7 integration feature.
The code is still there in 3.6b3pre, so you can still enable thumbnail previews in your about:config. The general feeling is that too many users were still seeing the spinning up circles and not actual thumbnails.
The general perception is that Windows 7 enthusiasts would be OK with simply flipping the switch themselves.
Ultimately, this move is about Firefox devs addressing more critical issues now so that 3.6 can be released. They'll have plenty of time to perfect the feature in future minor updates to 3.6. For now, the priority needs to be on getting 3.6 final out the door.