Observer don't get preferences change after extension upgrade
I've an extension for which, when upgraded, the preferences are lost
and the observer doesn't get the changes until restarting FF.
It happen on some FF installations (version 18.104.22.168/7 on WinXP) and
it is non deterministic, other PCs work fine.
The behaviour is the following:
- the user has a working and well configured extension
- the user upgrade the extensions (by opening from FF)
- the user lose the parameters and change them
- the observer inside the extension doesn't get any change
- after a restart, the extension use the changed parameters.