I've updated Lichess Additions — an extension for Firefox that I develop. It now includes the ability to annotate individual players with private notes, which will be stored locally on your computer.
The notes dialog will appear in the bottom left corner on Lichess, in the same vain as Lichess' online friends dialog in the bottom right corner. The dialog can be resized and pinned and can automatically pop up at the start of each game, showing the note (if any) about your opponent.
Notes can be added through the context menu (right-click) on players' names and annotated players' names will show a note icon.
Have a look and give it a shot:
I will update the extension's page with screenshots of this new functionality when I get the chance.
I hope you deem it useful. I look forward to your feedback.