So, this is a cute trick that’s been making the rounds:
In Firefox, right-click your bookmarks bar and pick “new bookmark”. Call it “Quick Notepad”, and in the Location box, put:
data:text/html,<html contenteditable>
and now when you click on that bookmark, your browser window will basically become Notepad, a very light text editor. File -> Save works great, too.
Perhaps better, if you check the “Load this bookmark in the sidebar” option, that will give you an nice little way of making notes about a tab, though unfortunately this option isn’t easy to save.