Well, I'm not sure if it's related to the Firefox update or the new Avast update (1.1.3237 dated 12/18/17), but I restarted Firefox with the add-ons disabled, closed the browser, and restarted again with add-ons enabled. I then updated to the newer Passwords extension and everything is working correctly again. I tried rebooting several times, and now the Avast Password extension works every time I open Firefox. I'm very happy! Thanks for the help!!