Actually this was reported previously on this thread, and we determined it was the result of a change from the recent ASP.NET security update that came down via windows update. The problem occurred if you had a persistent role cookie then it started throwing this exception right after the update was applied. It can be solved by clearing the cookies as you found, but I also patched the 184.108.40.206 release to handle this error and reset the cookie automatically. The reason it didn't happen in IE is probably because you didn't already have a persistent cookie there.
Thanks for the compliments on mojoPortal!