As the title says, FF seems to selectively forget cookies and thus requires me to constantly re-login.
I’ve had the exact same issue on two separate machines both running Ubuntu. My best guess is, that snap is at fault here, but I have no idea, why.
To reproduce the issue, I just have to perform the arcane ritual of “closing the app” and whoosh, cookies are gone. Plugins and settings persist, no “delete on close” option whatsoever is active. Vanilla Ubuntu shows exactly this behavior.
Not an Ubuntu user, but I think it’s all about how a snap uses filesystem, esp directories which are not writable by the “world”, such as your home directory.