This is a security measure Librewolf has. You can fix this in the settings.
Edit: Follow these steps to fix it
- Click on the padlock button on the URL bar at the page where you want to upload your image
- Click on "Connection secure" or whatever is written on the bar with another padlock icon
- More information > Permissions > uncheck "Extract canvas data" and select "Allow"
This will affect the current page only
Em outro comentário mandaram esse link.