Why does Shift+Insert paste from CLIPBOARD in some applications and PRIMARY in others?

How do I paste from the PRIMARY selection (eg. mouse-selected text) with a keyboard shortcut? Shift+Insert inconsistently pastes from PRIMARY or CLIPBOARD, depending on the application.

Background:

Ctrl+C copies selected text to CLIPBOARD while mouse-selection copies to PRIMARY. Paste from CLIPBOARD with Ctrl+V and paste from PRIMARY with mouse-middle-click.

In a terminal emulator (gnome-terminal), paste from CLIPBOARD with Ctrl+Shift+V. (Paste from PRIMARY with mouse-middle-click still.)

I want to paste from PRIMARY with a keyboard shortcut. In gnome-terminal, this is Shift+Insert, but in gedit and Firefox, Shift+Insert pastes from CLIPBOARD. I want a shortcut that consistently pastes from CLIPBOARD and a different short cut that consistently pastes from PRIMARY.

I'm running Ubuntu 14.04 with xmonad and Firefox 34.0