I use Zen everyday and a love it! I am glad they chose Firefox as a base, otherwise I would have skipped it. Firefox is stable, I open it when I boot my PC which runs for weeks and never think anything about it. On topic of ad blocking, I think that there are more ways to anoy users using ad blockers today despite of which browser someone uses, with ad block detection and blocking access. If your browser is build by a ad company, expect these changes. For this reason I won't use these browsers