Anyone with permissions to merge code in one of our repos can deploy to staging and production. Master is automatically deployed to staging, but production requires doing a manual promote to prod.
You should never be afraid of deploying: things break and that's okay! If you never did a rollback and think you will need assistance to do it, avoid deploying when less people are around (end of a work day or after-hours, Fridays, or weekends)... Unless circumstances require it. For example, to fix a crash bug after-hours.
If Heroku fails to build the app, it will not deploy it. However, if a runtime error is encountered and the app can't start up, a rollback should be performed.
For static apps deployed on AWS S3 that break, revert the bad commit and re-deploy.
Deployment notifications can be found for most apps in the #mofo-devops channel on the Mozilla Slack workspace