Firstly, I gotta point out that it is a fantastic idea. But it is not very user-friendly. I am using FireFox 3.6.13 on Windows 7 64-bit system, and clicking the pause button does nothing. In addition, the only two items showing up in tab context menu is "Pause Tab" and "Pause All Other Tabs". And shortcut to "Pause Tab" does not show up, but I tested my way to find out that it was Ctrl+Alt+P.
You should really add user customization abilities. Features like: "Unpause tab on focus", "Disable pause icon (on mouse hover)", "Custom shortcuts" would be highly appreciated. You could also consider integration with Tab Mix Plus, so that SaveMemory actions appear in the lists of Tab Mix Plus. (Things like middle-click tab="Pause Tab" could be possible through that.)