GoogleCalendar - CopyPaste is a firefox extention for google Calendar to copy, past and cut quickly every event of your calendars.
First select an event on one of your calendars, with right click or button combination CTRL+ALT+C o CTRL+ALT+X, after click on "copy" or "cut" the event to duplicate or move the event.
To paste the event select a date on displayed calendar and click, with right click or button combination CTRL+ALT+V, on "past in " and select the calendar when you want to copy the event . Therefore the event will be created on the selected date at same time of original event.
In order that the extension can run, it is necessary that user gives an explicit authorization to use his calendars, when the extension requires it.
The request will be required at most 10 times, this is the price to pay for a free release.