Automatically open ICS files in Google Calendar

I found a final solution to this! I based it off of the code to add Gmail as a mailto: handler, and adapted using information taken from Google Chrome's preferences file:

javascript:window.navigator.registerProtocolHandler("webcal","https://calendar.google.com/calendar/render?cid=%s","Google Calendar");

To use it, just go to Google Calendar, open Firefox's Web Console (Ctrl+Shift+K), and paste it into the code box.