[What's the secret to stayed cookied for PPRuNe then??]
On Firefox, Real Men hack a textfile that says "don't edit this textfile" and edit a number that's milliseconds since 1970 or somesuch.
Other people use the CookieEditor extension. You need to edit two cookies called bbuserid and bbpassword, make them expire whenever you feel like. I use 2015
On other browsers, I dunno.