Another casualty here. I have 8 addons (of about 25) uninstalled by this bug, including NoScript, Classic Theme Restorer, and Status4Evar, destroying Firefox usability. I'm running Avast Pro antivirus, v2014.9.0.2021. (On checking the Avast version, I was informed that "A new version of the program (2015.10.0.2208) is available", will update when I can.)
I'm curious whether any Avast Free users have experienced this bug.
My current best guess as to the problem scenario, run recent version of Avast antivirus and Firefox with Avast security plugin loaded, then upgrade through Firefox About menu, resulting in a number of addons being quietly uninstalled. (Avast and Malwarebytes indicate no malware on my system.)
Some advice, run
MozBackup immediately, so you can restore any addons subsequently lost. Additional advice - do NOT update Firefox until you have a backup made, or you may lose addons again! On seeing advice to upgrade Firefox, I did, to the latest (33.1.1), and discovered I had again lost the addons I had just re-downloaded and reinstalled.