A good outcome of this certificate fiasco is that many firefox users learned about the "studies" and "Normandy" brain-damage and promptly disabled them.
Having a capability for widespread A/B testing is brain-damage?
Are you the unlucky one that ended up with the broken A/B testing change? Well, sucks for you and nobody will be able to help you. Do you want to try a specific testing change because you like it/something is broken on your end and that will fix it? Sucks to be you again, you get no choice on this. Do you want to analyze the change to see if they have something bad in it? Nope, not possible unless if you end up being one of the lucky ones. In addition if Mozilla were to get compromised (either as an organization or in software side) could use it as a backdoor on unsuspecting users or even do targeted attacks.
None of that would not be an issue if they depended on nightly/beta for changes and error detection, nor would it be if they had a list of possible tests somewhere that the user could manually enable and possibly ask some users explicitly if they want to enable one on the browser startup.