There's another Arc-style navigation extension, but that one aims to fully replace the Firefox UI. That's a bit more than I wanted, so this one just adds a sidebar with some nice pinned tab UI in it. It's a light touch that can be hidden with no loss of functionality, which is all I wanted.
This plugin assumes that somewhere you have a folder of bookmarks called "Arc Tiles" and another called "Arc Pins". Any bookmarks in those will appear persistently on your sidebar. There's no UI to edit them, just edit the bookmarks and your sidebar should update.
