Have a way to show the sandbox has been restored successfully

[saidelike]

For the Firefox sandbox, atm we open Firefox once to initialize the profile folders and another time after we have restored the profile. For a new user, it may not be clear that until we saw this 2nd time Firefox, the sandbox is still not ready to be used.

Instead of calling Firefox.exe without paths, we could have a startup.html that just says Firefox restored successfully so it is clear for the user that it has finished restoring the sandbox.