Now just try unchecking the “cookie protection” box in the browser’s data protection settings
sorry mate, already unticked in firefox.
should i untick cookies in avast below
Yes, because it’s not the same thing.
This is just to see if it’s the new function that’s causing the problem
Password protection already existed before, but cookie protection has just appeared.
Since last post ive kept a tab open in firefox and everything stayed logged in.
just closed the tab and all logged out again.
unticked the cookie protect box in avast UI and all logged back in again so this was the culprit.
at least that saves having a tab open all the time.
problem solved but still cant understand how more people havnt been affected by this. may be due to staggered roll out around the world and some havnt got it yet or not installed it.
Oh well workaround is here now for others.
Thanks all for the replies.
Or they don’t encounter the problem at all. 
As I said, I’ve installed the update on 3 computers and I use Firefox essentially on all 3. I have no problem maintaining login on my various sites, even those that use 2FA (gmail, hotmail, …).
If you have the same problem with other browsers, it may be due to password manager (which I don’t use) or something that went wrong with the update.
In this case, if in a while no-one has come to complain about the same problem, you might want to consider reinstalling avast properly.
Have you tried avast repair? It doesn’t take long, so it might be worth a try.
Its a weird one. as no two setups are the same its trial and error until we find the solution.
I have nord vpn but i dont think its that.
as the unticking the cookie box in avast has totally solved the issue then im happy to roll with that for now and leave it unticked.
Thanks for all the help so far.
As I use Avast Free Antivirus and so can not use “Browser Data Protection” or “Cookie Protection” I wonder why I also suffer from that new introduced feature of being logged out of all tabs in Firefox after closing the browser.
I guess those parts of that new feature are activated even when Avast Free Antivirus is used.
And as there is no UI for those as they’re not “officially” activated I cannot fiddle with them.
This is my private PC, not a public library machine so please let me have my website cookies stay. I’m not a fan of incapacitation.
How to fix? Rollback Avast?
Edit:
Allright, it seems to be a one time occurance after the update of Avast and the reboot. But that never happened before that an Avast update deleted browser cookies.
I will observe this the next hours.
I was just about to say YAY im not the only one but then read your last bit. so hopefully a one off for you.
I have no idea if this new part is only in the paid version.
when you open avast on the left have you got a privacy tab on the left hand side. if so have a look in there.
Thanks to avast support there is a fix.
for some reason avast had added firefox to my blocked list.
removing from the blocked list and re ticking cookies im not logged out anymore.
https://forum.avast.com/index.php?topic=327453.msg1718929#msg1718929