https://bigwords.page/#smol!%20!%20!%20!%20!%20!%20!%20!%20!...!
You seem to have the same bug I had when I first implemented https://screaming.saej.in/
In Firefox, scrollWidth includes spaces that extend into the margin, so it's possible for text to fit perfectly on the screen, but scrollWidth > clientWidth. Fix here https://saej.in/posts/screaming/