We think we've figured out our CircleCI issues, so I've enabled the requirement that tests pass there in order for a pull-request to be mergable. We'll monitor this for problems, and you can always appeal to a repository admin (pmac, jgmize, agibson, or giorgos) for an override if necessary. Hopefully this will have the effect of fewer PRs merged that break the master branch build, but it could well end up being more trouble than it's worth. Feel free to let us know here or in #www on irc.mozilla.org
if you notice any issues.