I too was unable to access my emails, getting the same error the OP reported when using a bookmark. So i went to the Cox site and tried clicking on webmail from there, same error. I hopped in chat, was told to try a different browser, sure enough it worked in Chrome. Ok so its a Firefox issue. I asked the chat support if he could access emails in Firefox and he said they dont have the capability. HUH? He also wouldnt give me the link to the forums. Sigh.
I checked if Firefox updated recently and sure enough they released a high security update yesterday. So I went into Firefox Tools, Options, Privacy & Security and clicked on Manage Data under Cookies and Site Data. I then typed in Cox in the search box and clicked on Remove All Shown. Closed it out, retried emails and it worked.
I didnt have to reboot or close Firefox and I also run 3 addons (Adblock Plus, No Script and You No Cards) which had no effect on the situation as I turned them all off during testing. I turned them back on 1 at a time and made sure I could still access emails once the cookies were cleared and they didnt block anything. Im sure this is just a matter of Firefox forcing people to clear cookies for their high security update to be fully implemented.