For a few months now, it seems that I have to reenter my login credentials more and more often (it used to be once every few months, and it is now several times per week) when navigating Stack Overflow and other sites that share the same account (Travel, Sound Design, Expatriates, Signal Processing, and Video Production). I visit these sites almost daily.
I remember seeing some alert messages mentioning the need to load scripts from external URLs on the Stack Exchange websites recently, so maybe there's a link. (These message where solved by accepting to load from an alternative URL.)
In an answer to the questions about third-party cookies, there isn't any mention of such cookies any more in the Firefox privacy settings which would allow to explicitly set the permissions. Here is a list of cookies currently stored on my PC found on the stack domain:
stackapps 2 stackexchange 2 sound.stackexchange.com 2 stackoverflow.com 3
In addition, when on Travel SE, for example:
uBlock Origin does block one domain:
Firefox tracking protection does not block any domain.
When going to the Network Login Troubleshooting, I have a messaging failed information, and therefore a Communication with stackauth.com skipped information as well.
Unfortunately, as of today (December 20, 2020), there has not been a really convincing answer to the original question.
There seem to be a consensus that some interaction between the browser, it's privacy settings, the uBlock Origin extension and the server code is the cause of some unpredictable logging out for me.
If the issue was an impossibility to log in or a systematic log out, I would have tried some workaround that have been suggested here (disable ad-blocker, try another browser, ...) but given the erratic behavior of the problem, I don't expect these suggestions to give an answer to the why (other maybe than does not reproduce with such browser).
As I am currently moving from MacOS to Ubuntu, with the same browser settings and extensions, that will be an opportunity to see wether this has an impact on the issue.