hi,
i had the same problem since i tested all the wp 2.6 betas with firefox 3.
i searched a lot an i installed firefox again, but nothing helped me.
the solution in my case was verry simple!
i cleaned all cookies in firefox and then i noticed, if i go to the adminpanel with the login link in the frontend of the blog, i have no problems with login.
i always go to the dashboard with a direct bookmark in firefox that was https://myweblog.tld and my solution was, that i write www. before it. so the link in the bookmarks in firefox was now https://www.myweblog.tld and now i have no problems anymore.
if you look at the saved cookies in firefox, they all have www. before, and so i give a try to make the same in the bookmark link and it works.