For command chose Keys - Hotkey and in the Hotkey box press Ctrl + Shift + T.
The resulting command, exported, would look like this:
<VERSION>StrokeIt .9.4</VERSION>
<APP name="Firefox">
class=MozillaUIWindowClass
New Action {
gesture = A
New Command = keys, hotkey [CTRL_DOWN][SHIFT_DOWN]T[SHIFT_UP][CTRL_UP]
}
</APP>