Copy & Paste between terminal(s) and application windows in i3
Some terminals use CTRL+SHIFT+C and CTRL+SHIFT+V to copy and paste, while others use CTRL+ALT+C and CTRL+ALT+V, like urxvt (See https://wiki.archlinux.org/index.php/Rxvt-unicode#Cut_and_paste ).
For i3wm, try using a clipboard manager, like clipit.
sudo apt-get update
sudo apt-get install clipit
Add exec --no-startup-id clipit to ~/.i3/config to start it automatically.
I have just had success using the middle mouse button with xterm in i3 to copy from a remote terminal to firefox. Use the middle mouse button to copy and paste.