YouTube changed the layout causing the extension to replace thumbnails with blank spaces. I have fixed this in 2.1.7, but unfortunately Firefox is slow at reviewing updates.
This will take a few days to appear on the extension store. On Firefox, it could take up to 14 days, but you can switch to the beta version by clicking "FirefoxSignedInstaller" in the release (https://github.com/ajayyy/DeArrow/releases/tag/2.1.7). Make sure to backup your data and only have one extension enabled at once if you choose to do that.
While you wait on the old version, you can disable thumbnail replacements to see the original thumbnail instead of a blank space.
