Clipple keeps track of text you copy to the clipboard while using Firefox, and allows you to paste from that history instead of just the last item copied.
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.
Note: This download link takes you to the Mozilla.org add-on site, where you can automatically install the file into your Firefox browser.