I originally installed Firefox version 38 on Mac but later realized that I am expected to have only version 35 for my business. I downloaded Firefox 35. Uninstalled Firefox version 38 and installed version 35. As soon as I installed version 35, I went to preferences > Advanced > Update and set it to 'Never Check for Updates'. I closed the Firefox 35 instance and removed from Dock. Later I launched Firefox 35 and went to 'About Firefox' and noticed that browser instance started downloading new files and version 35 auto updated to version 38. I repeated same procedure at least 2-3 times but no luck. I also tried using about:config option and set preferences as follows: app.update.auto - false app.update.enabled - false app.update.silent - false But no luck. Can anyone advise how to get this Firefox downgraded to version 35?