If you need to copy several different pieces of text from several pages to different text boxes, cells or tabs, and spare the repetitive copy/switch/paste, grab MultiCopy, a clever Firefox extension developed by Guruprasad that allows you to copy as many as and as long pieces of text as you want and then paste them sequentially to wherever you can enter text. I tried with about 15 different pieces from several different tabs and it worked perfectly.
To copy the strings use the usual hotkey (Ctrl or Cmd + C) or the selection context menu. You can also take the different pieces to another application but you will get all of them pasted together. Not as handy but still helpful and time saving.
Try MultiCopy at Mozilla Add-ons.