FYI - This is not only a problem with Firefox. I encounter the same issue in both Chrome and Firefox. I spoke with Verizon Technical Support about the issue on Tuesday, December 19, 2011. They said they were not aware of a problem with either browser, but suggested some add-ons could cause the problem. Clearing all verizon-related cookies solves the problem for a single session only. Deleting the cookies for signin.verizon.com solves the problem for a single session, but upon logging out and trying to log back in the problem returns. Blocking cookies from signin.verizon.com is another way to recreate the same problem. A workaround which may help until it is fixed is to use Private Browsing sessions, which can be started with CTRL+SHIFT+N or CTRL+SHIFT+P. It is faster than deleting the cookie manually every time. :)