I cannot reproduce this problem with TenFourFox 45.9. This kind of problem existed before and was actually fixed with 45.9. If you still experience it with this version, you'll have to reset your profile:
Go to Help Menu>Troubleshooting Information>Refresh TenFourFox… (Informations de dépannage>Réparer TenFourFox) and follow instructions. Then re-install Qwant.