the description said "App brings Twitter™ right into your Firefox Sidebar. "

this is only truth for the older version. New version offer NO such feature and it is very annoying.

----UPDATE : 2019-08-28-------
They have fixed the issue and it works in the side bar now YAY! the only bad thing now is that twitters new ui looks bad with the side bar.

Great app but i do miss the way it used to work with the bar on the side. Maybe add an option to change where you want it like if you want it to be a floating window or a bar on the side of your screen,