This is a deliberate change in Thunderbird to support a feature offered by some mail providers. This allows a user to create a unique personal address for each correspondent. Sometimes it is called it a "throw-away" address. A possible reason for using this is the notion that if a specific address is used by a third party you could claim that the original correspondent sold on your personalised address.

You appear to have an email provider who permits arbitrary sender addresses. Many (most?) email providers do not support this feature. Gmail does; search for their '+' feature in email addresses. I think this particular option is rather lame, as it is trivially easy to identify the true address that the so-called throw-away address is based on.

I suppose you mean the option to customize the from address from the Compose window. There is no preference to remove this option from the user interface. If you don't want the user to have this option, the best you can do, is hide it from the user interface, but this isn't straightforward. Creating a folder called chrome in your profile directory and create a file called userChrome.css in a it. (The filename must match exact, caps included.) Then add the following lines to the userChrome.css file:

#msgIdentityPopup:last-child,
#msgIdentityPopup:first-child:nth-last-child(1)  {
  display: none;
}

After restarting Thunderbird, the last two items from the down-down list are hidden.