Yesterday I downgraded my browser from 53.0 to 44.0.2 due to incompatibilities with extensions. I deleted the .app file along with 3 directories within my user profile, then a fresh install of 44.0.2. I immediately went into Preferences and disabled the upgrade check option completely. This morning I logged in to my Mac and received a message about the upgrade back to 53. Not sure why this would happen if the upgrade option is disabled. Would Firefox Sync re-enable the option somehow? I went back to Preferences and checked the Upgrade option and it is clearly disabled.