Still giving this 5 stars but just wanted to let others know that the latest update, 3.9, completely stops Firefox loading any sites (for me at least).
This will obviously be fixed as it's only been out for a few days, but if this is happening to you too, you'll need to completely remove Adblock and reinstall the previous version, 3.8.4, while also disabling automatic updates.
Everything's working again now, although whitelisted sites list was cleared of course.