alt Hacker NewsPassword managers often go through quite some hassle to keep passwords 'safe' in memory. However, I often do not get the attack model of many of those tools. Tools like keepass e.g. go through quite to register a browser plugin. But then anyone with normal user rights can extract that key from the browser and do everything with it. Also this whole 'trust this browser' stuff of web apps seems strange if one e.g. can read the cookie store easily...
Cookies, if done correctly, will store a string that the server offered after a successful authentication - that string should have nothing to do with the password (it might contain some user information for logging/cross site tracking) but nothing sensitive.
With said cookie you can absolutely impersonate a user for while (potentially needing to evade user agent string checks and the like but often not)... but it will expire and then your access should be ended. If the site is well designed actions like password changing should also re-require the user's password instead of allowing anyone with just the cookie from proceeding with the action.
If it is done right cookies are pretty decently secure at keeping your secrets safe but, for convenience they do lower the security that could be accomplished with more involved techniques.
As an aside Oauth's key -> token approach is basically identical to password -> cookie (assuming best practices are in place).