Doesn't work & isn't needed anymore. Just type "about:processes" (without quotes) into the address bar and you get all the RAM info about every tab without needed an extension.

not working

As of v1.0, this is just a stub extension ( name: __MSG_extensionName__, description: __MSG_extensionDescription__?! xD) that will only display the user-agent string and system architecture when clicking on the button.
So no alternative to the firefox task manager (CTRL+ESC) yet.
@annette: if you open source the ext we can improve it together : )