I'm running 1.9.1 beta and have a similar problem like [url=http://forums.b2evolution.net/viewtopic.php?t=10114]this[/url].
Additionally there seems to be a problem with Firefox' SessionHandling with the Evo-Cookies. When restarting the browser i'm prompted to login but i can't i always get back to the login form. I then have to close the browser another time and reopen it to make it working.
This only happens with B2E... strange thing.
any idea?
temporary solution: https://addons.mozilla.org/firefox/1595/
this addon let's you delete cookies just for one site via the context menu. After deleting the b2e-cookie it works fine.