There's no reason why this would 'disappear'. It's not a tab it's s drop-down menu for changing the format. You've not really given much information about the problem. You can start by updating your
forum profile with the output of the following command (do not post the output in this thread and get your admin to run the command for you):
Which browser? Which operating system? By saying that it's 'disappeared' you imply that it was there once upon a time, what has changed? Does it happen for all users or just one user? has it disappeared for sending a Reply as well? Does this problem happen when using a different browser? Have any add-ons been added to the browser being used?