Gmail's new mandatory categories is extremely annoying and needs to be disabled

Some make changes for improvements. 
" " " for bug fixes.
" " " for new features.
and some makes changes for the sake of change itself.  Just to make things different.

That's what Gmail did. They took an existing feature that everyone has grown accustomed to and forced additional categories in their effort to place ads ("promotions" was the primary label that Google wanted to throw in along with bundling it with "Social", "Updates" and "Forums") into the inbox.

Even if you configure the inbox to disable/hide and undo this intrusion, you are still stuck with the minor annoyance of scrolling a bit on the dropdowns that appear in the "Move To" and "Labels" buttons

You can't disable (yet) these drop-downs that appear but if you use Firefox with Adblock you can hide them thanks to the fine people at Google Groups who made the effort complain and a user (Mario Krušelj) to post a custom solution.

In Firefox, edit (or create if you dont have) the category "Element Hiding Helper" in your filter preferences and add the following filters:

mail.google.com##.J-N[title="Social"]
mail.google.com##.J-N[title="Promotions"]
mail.google.com##.J-N[title="Updates"]
mail.google.com##.J-N[title="Forums"]
mail.google.com##.J-LC[title="Social"]
mail.google.com##.J-LC[title="Promotions"]
mail.google.com##.J-LC[title="Updates"]
mail.google.com##.J-LC[title="Forums"]

Screenshot below: