Make sure default Firefox themes are enabled on ESR68

With current 000-tor-browser.js prefs default theme extensions are not installed. AddonManager.maybeInstallBuiltinAddon is used to install these, which requires enabledScopes to have flag 4 (application scope). So changing extensions.enabledScopes from 1 to 5 fixes this.

My understanding is that this should only allow installing extensions from application directory, which we control. Is there any reason not to do this change?

Also, some related prefs seem obsolete:

extensions.bootstrappedAddons
extensions.enabledAddons
extensions.enabledItems

We could also use this to clean them up.