Tab Mix Plus
Browser tabs are darned handy, but they could do more. With Tab Mix Plus, Firefox tabs go past the obvious and into the indispensable.
The first feature you see is TMP's session restore. Yes, Firefox comes with its own tab-saving option, but Tab Mix Plus does it one better by letting you restore either your last batch of open tabs or the batch before that. When you open Firefox and use TMP's session restoration, TMP italicizes the unvisited tab titles in red, making it easy for you to tell which one's you've seen recently. You can customize the styles, if you prefer.
Tab Mix Plus's options menu offers you a huge array of tweaks to make Firefox work the way you think. You can set new tabs to open as blank, as your home page, or as a duplicate of the current page. You can set bookmarks to open in new tabs instead of overwriting the tab on top. You can enable all sorts of mouse functions for tabs, such as selecting a page by hovering the mouse on the tab.
Once you get past the sheer breadth of features, TMP is easy to configure. And once you start making the changes, it's hard to imagine how you lived without it.
Note: This link takes you to Mozilla.org, where you can automatically install the file into your Firefox browser. This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.