It was only the other day that I noticed that in Windows 7, Firefox (or the beta that I run of Minefield) does not show multiple tab previews that Internet Explorer 8 (or Internet Explorer 9 beta as I’m running on my netbook) does.
So a quick search of the internet and looking down the about:config list in Firefox holds the clue, the taskbar preview option is set to false (disabled), so toggling this option to true (enabled) sets the preview in the taskbar.
I took a few screenshots of the steps as a very quick guide below, do be warned that editing the about:config menus in Firefox can wrongly can cause Firefox to fail.
Open Firefox (or Minefield) and in the address bar type about:config and hit enter
Once the list of options are open, its best to filter the list down and to do this in the filter box type browser.taskbar.preview and the list will narrow down to only three, the one you need to highlight is browser.taskbar.preview.enable and then right click it to give you the option to Toggle the setting from false to true
Once set to true, restart Firefox and now when you have a few tabs open you will get the mini previews of the open windows in the taskbar.