When a user is not logged in and tries to reply to a post if the login fails and the user clicks back (browser button or link) the submit button is disabled.
I have found a workaround that will make it work correctly in FireFox though. Confirmed fix for FireFox 3.5.5
Just look for this code in your post.asp file.
if strRqMethod = "Forum" or _ strRqMethod = "URL" or _ strRqMethod = "EditURL" or _ strRqMethod = "EditForum" then if strPrivateForums <> "0" then if mLev = 3 then Response.Write " onclick=""selectAllOptions(document.PostTopic.AuthUsers);""" else Response.Write " onclick=""selectAllOptions(document.PostTopic.AuthUsers);selectAllOptions(document.PostTopic.ForumMod);""" end if else if mLev > 3 then Response.Write " onclick=""selectAllOptions(document.PostTopic.ForumMod);""" end if end if Response.Write ">"
This will ensure that the button is always enabled no matter what browser you are using. And it still disables while submitting so user should not be able to click it twice if it submits a little slowly.