I have the ~exact~ same problem here. Given that security experts universally suggest that users should fully clear browsers on exiting, it's surprising that anyone would use cookies to hold important setup information which needs to persist from session to session, from day to day. Let's hope they fix this soon. Until then, I'm going to disable the Firefox plugin and hope this works......
That's an exaggeration. For many years I have followed a suggestion once found in a security forum to allow and keep site cookies, but reject third party cookies. Used in conjunction with adequate security programs, always including SpywareBlaster, that has proved more than enough for me. MalwareBytes scans occasionally throw up one or two doubtful cookies, but never anything worse.
On the other hand, while I consider Avast to be a fine AV, the AOS Firefox plugin is more trouble than it is worth. Hell will freeze over before I enable it again. The majority of posts here in recent times have been regarding AOS rather than Avast. If you want the website ratings, use WOT instead.