When Firefox on Windows 10 doesn't know how to open a file, Firefox will use a placeholder, "TWINUI," in the "Open with" drop-down. This typically happens after you've installed the OverDrive app (the app store version or desktop version), but haven't downloaded a title to it yet.
If you see this prompt:
- Click the drop-down menu, and choose the OverDrive app from the list.
- If you don't see the OverDrive app listed, choose Other... to find it.
- Click OK.