The tiny, 24KB add-on doesn't require any setup. After a quick install, just right-click in any part of a page where you can type text - a search box, say, or an online document - and select the new "Clipple - Paste" option. Then paste in your pick of the last 15 things you copied to the clipboard.
Clipple adds a new clipboard icon to the lower-right of the browser's Status Bar. This allow you to change a few options, such as changing the number of items to save or raising a particular item to the top so it can be pasted with Ctrl-V. By default, Clipple will save your clipboard when you close and re-open Firefox, but you can change that behavior as well.
The handy Clipple would be all the better if you could quickly assign a keyboard shortcut to its extended paste. Its author suggests using one of a few different add-ons such as KeySnail or Vimperator to set up a shortcut, but doing so would be a fair bit more complex than using the dead-simple Clipple. See the add-on's page for more details.
Still, when many of us are using the browser for tasks we used to do with multiple different programs, Clipple adds a nice new feature. Even without a shortcut.