TooManyTabs (free/donationware) is for the power-browser who likes to keep many Firefox tabs open at once. Instead of keeping all those tabs active, which can eat up your computer's memory, TMT creates a sort of waiting area where tabs can sit--still visible, but using no memory.
The add-on installs a new toolbar in Firefox above the normal tabs. If you drag-and-drop a tab into the TooManyTabs toolbar, or click a stylized up arrow on the left side of the active tab, it moves up to the TMT toolbar.
A tab in the TooManyTabs toolbar lies dormant, and doesn't use any computer memory. Instead, the URL for that tab sits in a new bookmark folder tucked away in the Unsorted Bookmarks folder. If you click a TMT tab, the add-on quickly restores the page to the regular tab bar using that saved URL and your browser's disk cache.
You can use up to six rows to store your background tabs, and can switch between rows in the toolbar or view multiple rows at once. A TooManyTabs tutorial video at the developer Visibo's site can help you get the most out of using multiple rows. Visibo also developed the useful Incredible Bookmarks add-on.
TooManyTabs offers a nice halfway point between keeping a tab active and using up memory, and filing it away in a bookmark where you might forget about it. If you tend to keep many tabs open at once for quick reference, the handy add-on can help conserve your computer's resources.