Open tabs on the right.

In Firefox 3.6, the default behavior is that when you open a new tab from a link on the page, it appears just to the right of the active tab.  The old behavior was to have it appear at the very end of your list of tabs.  To get the old behavior back, go to >about:config and set the value browser.tabs.insertRelatedAfterCurrent to false.

Ctrl+tab previews.

To get an about:config and set browser.ctrlTab.previews to true.  Note, you can press Ctrl+Shift+Tab to show the expanded list of tabs, Ctrl+Tab just shows your most recent ones.

Windows 7 taskbar previews of all tabs.

If you are using Windows 7, you can go to about:config and set browser.taskbar.previews.enable to true.  Now, the Windows 7 taskbar will have a preview for each tab, rather than just one for each Firefox window.  (Note that this feature works off-and-on for me, I think that it’s probably not completely finished which is why it is not yet on by default.)