That’s becoming really annoying. You answer thread as logged in user (as long as you can believe what forum tells you), spend half an hour to carefully write message with links, quotations, images and what not, press “submit” or “advance”, suddenly find itself in login dialog, log in and find yourself on 404 not found page. Half on hour work is lost.
Not to mention excellent message from Firefox “redirecting loop detected” when trying to create new post just after having logged in.
The only way to make it working again was to exit firefox (with all other windows and tabs open!) and start again.
But you should see the advantage of this method, because there is a very nice feature behind that “bug”. It usually hits you when you start being vulgar, some kind of artificial self-censorship, very efficient indeed (it prevented me from getting nasty a couple times).
The short answer to your question is “never”.
The long answer is explaining to you how cookies work and maybe suggesting to copy/paste from a text editor.
My advice (nothing religious meant though) is to read/re-read/re-re-read the book of Ecclesiastes and aknowledge that all is Vanity.