Webpage SideNotes oleh Nabendu

This firefox addon adds a sidebar that lets you take notes on the current web page. The notes are saved to local storage, and the notes for each page are shown again when you open that page again.

Open the sidebar from View -> Sidebar -> SIdeNotes
The sidebar can also be open using Ctrl+Shift+Y in WIndows and Cmd+Shift+Y in Mac.