List.it is a no frills notes keeper that resides inside Firefox’s sidebar. It is simple and open source and provides an option to synchronize notes but they are kept locally if the user does not use or ignores the sync option.
Install the add-on and access the sidebar interface via Ctrl+Shift+F. Enter any text and save, access the settings by clicking the gear icon. By default the top line of a note is shown, click the arrow icon to expand all. Click x if a note needs to be deleted and use the garbage can icon to view the dustbin