Firefox 2 has a function to restore closed tabs

Right-clicking on the tab will bring up a menu titled "Undo Close Tab" which will allow you to restore the closed tabs to their original state before closing. It is a bit different but it seems to be quite useful.

Note that this function has already been implemented in Opera. Details are as follows.
Firefox 2.0 Gets Undo Close Tab & Scrolling Tabs

For Opera, it is easy to understand with the feeling like the image below, and you can restore the closed tab by choosing "Restore". It is surprisingly convenient.