make addon restartless

Main motivation is better UX when tlsnotary ships as addon in the future.
Restartless addons can also be debugged right in the browser (non-restartless could not)
No need to make a Firefox copy anymore because we do not use the distribution/bundle method now.
(IIRC we used bundling to quietly install the addon. However, the fact that the addon is now restartless does not mean that it will install quietly. I had to resort to a trick in start_firefox() )

Also:
test suite passed on all 3 platforms