I *think* that is possible. Keep in mind that there will be a longer delay in the resizing until Firefox alerts the add-on of the URL being loaded in the tab.

There are quite a few tasks to work through to make it a general purpose solution: (1) currently the add-on doesn't have permission to read URLs, but that could be added as an optional permission for those who want to make lists; (2) there needs to be some convenient way to specify the URL (for example, toolbar button or right-click context menu item); and (3) there needs to be a way to remove items from the list.

I don't know how soon I would be able to work on that. Meanwhile, you might ask the author of the other extension whether they could add a feature to resize their window on launch.