Similar to this question, is it possible to update installed plugins via the command line?

I'm running YSlow with beacon reporting as a nightly cron job under OSX:

/Applications/Firefox.app/Contents/MacOS/firefox-bin -no-remote -P YSlow http://www.example.com/ &
PID=$!
sleep 300
kill $PID

This dumps FF into the background and grabs the PID, waits 300 seconds (for the page to load) then kills it. If there is an update pending, the browser "hangs" waiting for a confirmation.

If I do click on the "install updates" link, everything works and then Firefox launches a new process - the $! returned by the shell is no longer valid.

Can I update a plugin from the command line without confirmation? Can I curl the XPI into a file and install it without confirmation?