I haven't encountered this myself, but I found an old forum post that says they resolved it by reinstalling Firefox. That's probably not necessary though (refreshing Firefox without uninstalling should be good enough).

If you've already followed all the steps to unblock cookies and it still doesn't work then refreshing Firefox might solve the problem.

I have tried several times to refresh Firefox but I cannot find anything labeled "Refresh..." to click. Where is it?

This issue is resolved: I backed up my bookmarks, then completely erased Firefox Portable app and all its folders, then reinstalled the app and restored my bookmarks. Works OK again.