You can get the full thumbnails in Firefox in Windows 7 just like Internet Explorer 8.

1. Navigate to about:config.
2. If prompted, confirm that you know what you’re doing.
3. In the filter text field type browser.taskbar.previews.enable.
4. Double click the entry that appears in the list. The value should change from false to true.
5. Hover your mouse over the taskbar tile to see the expanded thumbnails.

Nice.

Thanks to Ed Bott for posting this tip!