In every single case you've described the attacker can:
a) Already ready your passwords from memory/ webpages/ any other of the million ways
b) Access your cookies and session tokens
If the attacker is in a position to read the plaintext file off of the disk it really won't matter if it's encrypted. You're welcome to do so, I'm sure there are extremely niche scenarios where it may help, but it's not really worth mentioning imo.