This is a great add-on, just what I was looking for! Euan, could you make opening a saved list a bit easier. Now, I press the add-on button, then select the list, then press the Load button, then press Open URLs button - 4 actions. You can make it 2: 1) press the add-on button 2) select a list - and the tabs start opening. Much quicker, eh?
There are several options available for you to reduce the amount of clicks to open lists:
1. Open settings (Cog in top right of popup), then check "Automatically open lists", save the settings. Then when you click on a list from the dropdown menu, it will automatically open.
2. Use context menus (Currently experimental feature). Open settings (Cog in top right of popup), then check Enable context menus (Right click menus), save settings and then restart your browser. Then once you relaunch your browser, you can right click and will have a menu option for opening lists.
It is not currently possible to make tabs open in the background, however this feature can certainly be added in the next version of the addon (which is currently in development, and should hopefully be out in the near future).
Edit: You should be able to have tabs open without being selected. Just make sure that "Open tabs 'Active'" is disable in the settings.
It is certainly something that could be worked on for a later release of the addon.
Edit 18/11/19: Version 1.5.1 has now been released. This update brings in the feature you mentioned in this review. In the settings page, you can now choose how non-url strings will be handled by the addon.