I use the browser full screen.
Every-time I try to press the back button, the Ubuntu side menu pops up.
Some people told me I need to do it very very fast, and it will work, but I am somewhat disabled...can't do it fast.
Is there a way to put that bar down, where it was before?
In Firefox it's Alt + ← , in Chrome it's Backspace, for example.
This question has an answer explaining how to stop the launcher from autohiding.
This one explains how to adjust the delay so the launcher can pop out very quickly or slowly, as you like.
Both of the linked questions have you installing and configuring CompizConfig Settings Manager, and adjusting the Unity Plugin. Be very careful making adjustments if this is your first time using this tool. Don't un-check the 'Enable Ubuntu Unity Plugin' box by accident, or change anything you aren't sure about. You can quickly leave yourself without a usable system if you make a mistake. It's a great tool, just be cautious.