I had to log in today, but only once after starting up a multisession FF, and it worked for all 4 forums in 2 windows.
What is far worse for me is the change from FF21 to FF22 which has increased the text size on all websites, and also the widths in these forums. Well, maybe it is actually better this way, with the things at the side being almost out of view, so I can only see the left part of them, such as Start a Discu (enough to click it) and Top Part and so on.